[media] v4l: Don't register media entities for subdev device nodes

Subdevs already have their own entity, don't register as second one when
registering the subdev device node.

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