scsi: hisi_sas: Fix NULL pointer exception during user_scan()

user_scan() invokes updated sas_user_scan() for channel 0, and if
successful, iteratively scans remaining channels (1 to shost->max_channel)
via scsi_scan_host_selected() in commit 37c4e72b0651 ("scsi: Fix
sas_user_scan() to handle wildcard and multi-channel scans"). However,
hisi_sas supports only one channel, and the current value of max_channel is
1. sas_user_scan() for channel 1 will trigger the following NULL pointer
exception:

[  441.554662] Unable to handle kernel NULL pointer dereference at virtual address 00000000000008b0
[  441.554699] Mem abort info:
[  441.554710]   ESR = 0x0000000096000004
[  441.554718]   EC = 0x25: DABT (current EL), IL = 32 bits
[  441.554723]   SET = 0, FnV = 0
[  441.554726]   EA = 0, S1PTW = 0
[  441.554730]   FSC = 0x04: level 0 translation fault
[  441.554735] Data abort info:
[  441.554737]   ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000
[  441.554742]   CM = 0, WnR = 0, TnD = 0, TagAccess = 0
[  441.554747]   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
[  441.554752] user pgtable: 4k pages, 48-bit VAs, pgdp=00000828377a6000
[  441.554757] [00000000000008b0] pgd=0000000000000000, p4d=0000000000000000
[  441.554769] Internal error: Oops: 0000000096000004 [#1]  SMP
[  441.629589] Modules linked in: arm_spe_pmu arm_smmuv3_pmu tpm_tis_spi hisi_uncore_sllc_pmu hisi_uncore_pa_pmu hisi_uncore_l3c_pmu hisi_uncore_hha_pmu hisi_uncore_ddrc_pmu hisi_uncore_cpa_pmu hns3_pmu hisi_ptt hisi_pcie_pmu tpm_tis_core spidev spi_hisi_sfc_v3xx hisi_uncore_pmu spi_dw_mmio fuse hclge hclge_common hisi_sec2 hisi_hpre hisi_zip hisi_qm hns3 hisi_sas_v3_hw sm3_ce sbsa_gwdt hnae3 hisi_sas_main uacce hisi_dma i2c_hisi dm_mirror dm_region_hash dm_log dm_mod
[  441.670819] CPU: 46 UID: 0 PID: 6994 Comm: bash Kdump: loaded Not tainted 7.0.0-rc2+ #84 PREEMPT
[  441.691327] pstate: 81400009 (Nzcv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
[  441.698277] pc : sas_find_dev_by_rphy+0x44/0x118
[  441.702896] lr : sas_find_dev_by_rphy+0x3c/0x118
[  441.707502] sp : ffff80009abbba40
[  441.710805] x29: ffff80009abbba40 x28: ffff082819a40008 x27: ffff082810c37c08
[  441.717930] x26: ffff082810c37c28 x25: ffff082819a40290 x24: ffff082810c37c00
[  441.725054] x23: 0000000000000000 x22: 0000000000000001 x21: ffff082819a40000
[  441.732179] x20: ffff082819a40290 x19: 0000000000000000 x18: 0000000000000020
[  441.739304] x17: 0000000000000000 x16: ffffb5dad6bda690 x15: 00000000ffffffff
[  441.746428] x14: ffff082814c3b26c x13: 00000000ffffffff x12: ffff082814c3b26a
[  441.753553] x11: 00000000000000c0 x10: 000000000000003a x9 : ffffb5dad5ea94f4
[  441.760678] x8 : 000000000000003a x7 : ffff80009abbbab0 x6 : 0000000000000030
[  441.767802] x5 : 0000000000000000 x4 : 0000000000000000 x3 : 0000000000000000
[  441.774926] x2 : ffff08280f35a300 x1 : ffffb5dad7127180 x0 : 0000000000000000
[  441.782053] Call trace:
[  441.784488]  sas_find_dev_by_rphy+0x44/0x118 (P)
[  441.789095]  sas_target_alloc+0x24/0xb0
[  441.792920]  scsi_alloc_target+0x290/0x330
[  441.797010]  __scsi_scan_target+0x88/0x258
[  441.801096]  scsi_scan_channel+0x74/0xb8
[  441.805008]  scsi_scan_host_selected+0x170/0x188
[  441.809615]  sas_user_scan+0xfc/0x148
[  441.813267]  store_scan+0x10c/0x180
[  441.816743]  dev_attr_store+0x20/0x40
[  441.820398]  sysfs_kf_write+0x84/0xa8
[  441.824054]  kernfs_fop_write_iter+0x130/0x1c8
[  441.828487]  vfs_write+0x2c0/0x370
[  441.831880]  ksys_write+0x74/0x118
[  441.835271]  __arm64_sys_write+0x24/0x38
[  441.839182]  invoke_syscall+0x50/0x120
[  441.842919]  el0_svc_common.constprop.0+0xc8/0xf0
[  441.847611]  do_el0_svc+0x24/0x38
[  441.850913]  el0_svc+0x38/0x158
[  441.854043]  el0t_64_sync_handler+0xa0/0xe8
[  441.858214]  el0t_64_sync+0x1ac/0x1b0
[  441.861865] Code: aa1303e0 97ff70a8 34ffff80 d10a4273 (f9445a75)
[  441.867946] ---[ end trace 0000000000000000 ]---

Therefore, set max_channel to 0.

Fixes: e21fe3a52692 ("scsi: hisi_sas: add initialisation for v3 pci-based controller")
Signed-off-by: Xingui Yang <yangxingui@huawei.com>
Signed-off-by: Yihang Li <liyihang9@huawei.com>
Link: https://patch.msgid.link/20260305064039.4096775-1-liyihang9@huawei.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>