media: v4l2-subdev: fix regression in check_pad()

sd->entity.graph_obj.mdev can be NULL when this function is called, and
that breaks existing drivers (rcar-vin, but probably others as well).

Check if sd->entity.num_pads is non-zero instead since that doesn't depend
on mdev.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reported-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Fixes: a8fa5507 ("media: v4l2-subdev: Verify arguments in v4l2_subdev_call()")
Tested-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/v4l2-core/v4l2-subdev.c

@@ -124,7 +124,7 @@ static inline int check_which(__u32 which)
 static inline int check_pad(struct v4l2_subdev *sd, __u32 pad)
 {
 #if defined(CONFIG_MEDIA_CONTROLLER)
-	if (sd->entity.graph_obj.mdev) {
+	if (sd->entity.num_pads) {
 		if (pad >= sd->entity.num_pads)
 			return -EINVAL;
 		return 0;