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

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: a8fa55078a77 ("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>