[media] omap4iss: change the logic that checks if an entity is a subdev

As we're getting rid of an specific number range for the V4L2 subdev,
we need to replace the check for MEDIA_ENT_T_V4L2_SUBDEV by a macro.

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