drivers: rtl8723bs: rewrite comparison to null

fix following post-commit hook checkpatch warnings:

CHECK: Comparison to NULL could be written "!psta"
97: FILE: drivers/staging/rtl8723bs/core/rtw_ap.c:2115:
+		if (psta == NULL)

Signed-off-by: Fabio Aiuto <fabioaiuto83@gmail.com>
Link: https://lore.kernel.org/r/0c6d53c851d1b07eb0183108e0bad7b4f273f04b.1617802415.git.fabioaiuto83@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>