mfd: ls2kbmc: Use PCI API instead of direct accesses

There is a PCI API to access device resources. Use it instead of
direct accesses.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Binbin Zhou <zhoubinbin@loongson.cn>
Link: https://patch.msgid.link/20251030113735.3741913-3-andriy.shevchenko@linux.intel.com
Signed-off-by: Lee Jones <lee@kernel.org>