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: NFabio Aiuto <fabioaiuto83@gmail.com>
Link: https://lore.kernel.org/r/b32610045cad3c385b236266d7a10665ed202150.1617802415.git.fabioaiuto83@gmail.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8723bs/os_dep/mlme_linux.c

@@ -148,7 +148,7 @@ void rtw_report_sec_ie(struct adapter *adapter, u8 authmode, u8 *sec_ie)
 	buff = NULL;
 	if (authmode == WLAN_EID_VENDOR_SPECIFIC) {
 		buff = rtw_zmalloc(IW_CUSTOM_MAX);
-		if (NULL == buff)
+		if (!buff)
 			return;
 
 		p = buff;