[media] v4l2-subdev: use MEDIA_ENT_T_UNKNOWN for new subdevs

Instead of abusing MEDIA_ENT_T_V4L2_SUBDEV, initialize
new subdev entities as MEDIA_ENT_T_UNKNOWN.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>