[media] v4l: Release module if subdev registration fails

If v4l2_device_register_subdev() fails, the reference to the subdev
module taken by the function isn't released. Fix this.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: stable@kernel.org
Acked-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>