staging: rt18188eu: Remove useless cast on NULL

Cast on NULL to a pointer type is not required. This
patch removes NULL casted to some pointer type.

The sematic patch used is:
@r@
type T;
@@
- (T *)NULL
+ NULL

Build tested it.

Signed-off-by: Tapasweni Pathak <tapaswenipathak@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8188eu/core/rtw_mlme_ext.c

@@ -945,7 +945,7 @@ unsigned int OnAssocReq(struct adapter *padapter, struct recv_frame *precv_frame
 	}
 
 	pstat = rtw_get_stainfo(pstapriv, GetAddr2Ptr(pframe));
-	if (pstat == (struct sta_info *)NULL) {
+	if (pstat == NULL) {
 		status = _RSON_CLS2_;
 		goto asoc_class2_error;
 	}