[media] v4l: vdev_to_v4l2_subdev() should have return type "struct v4l2_subdev *"

vdev_to_v4l2_subdev() should return struct v4l2_subdev *, not void *. Fix
this.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>