[media] s5c73m3: fix subdev type

This sensor driver is abusing MEDIA_ENT_T_V4L2_SUBDEV, creating
some subdevs with a non-existing type.

As this is a sensor driver, one of the entries is
MEDIA_ENT_T_CAM_SENSOR. The other one will be using
MEDIA_ENT_T_V4L2_SUBDEV_UNKNOWN, because the subdev is not
any of the already existing types.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>