staging: rtl8723bs: fix comparison in if condition

fix following post-commit checkpatch issue:

WARNING: Comparisons should place the constant on the right side of the test
1833: FILE: drivers/staging/rtl8723bs/os_dep/mlme_linux.c:151:
+		if (NULL == buff)

Signed-off-by: Fabio Aiuto <fabioaiuto83@gmail.com>
Link: https://lore.kernel.org/r/b32610045cad3c385b236266d7a10665ed202150.1617802415.git.fabioaiuto83@gmail.com


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