wifi: wl1251: validate packet IDs before indexing tx_frames

wl1251_tx_packet_cb() uses the firmware completion ID directly to index
the fixed 16-entry wl->tx_frames[] array. The ID is a raw u8 from the
completion block, and the callback does not currently verify that it
fits the array before dereferencing it.

Reject completion IDs that fall outside wl->tx_frames[] and keep the
existing NULL check in the same guard. This keeps the fix local to the
trust boundary and avoids touching the rest of the completion flow.

Signed-off-by: Pengpeng Hou <pengpeng@iscas.ac.cn>
Link: https://patch.msgid.link/20260323080845.40033-1-pengpeng@iscas.ac.cn
Signed-off-by: Johannes Berg <johannes.berg@intel.com>