staging: rtl8723bs: fix null check conditions in core/rtw_cmd.c

fix post-commit hook checkpatch issues:

CHECK: Comparison to NULL could be written "!psta"
178: FILE: drivers/staging/rtl8723bs/core/rtw_cmd.c:2012:
+			if (psta == NULL)

CHECK: Comparison to NULL could be written "!psta"
198: FILE: drivers/staging/rtl8723bs/core/rtw_cmd.c:2066:
+	if (psta == NULL)

CHECK: Comparison to NULL could be written "!psta"
211: FILE: drivers/staging/rtl8723bs/core/rtw_cmd.c:2081:
+	if (psta == NULL)

Signed-off-by: Fabio Aiuto <fabioaiuto83@gmail.com>
Link: https://lore.kernel.org/r/03e36e1710ea4dfd07c3753d9a8df5746e59bc18.1617545239.git.fabioaiuto83@gmail.com


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