[media] s5k5baf: fix subdev type

The driver creates two subdevs, one for the image sensor pixel array
(and the related readout logic) and one for an ISP.

The first subdev already uses the MEDIA_ENT_T_V4L2_SUBDEV_SENSOR type,
but the second subdev isn't a sensor pixel array.

So, rename the second subdev as MEDIA_ENT_T_V4L2_SUBDEV_UNKNOWN.

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