video: mmp: Using plain integer as NULL pointer

Sparse complains here:

	drivers/video/mmp/core.c:33:16:
		warning: Using plain integer as NULL pointer
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

drivers/video/mmp/core.c

@@ -30,7 +30,7 @@ static struct mmp_overlay *path_get_overlay(struct mmp_path *path,
 {
 	if (path && overlay_id < path->overlay_num)
 		return &path->overlays[overlay_id];
-	return 0;
+	return NULL;
 }
 
 static int path_check_status(struct mmp_path *path)