[media] staging: omap4iss: get entity ID using media_entity_id()

Accessing media_entity ID should now use media_entity_id() macro to
obtain the entity ID, as a next patch will remove the .id field from
struct media_entity .

So, get rid of it, otherwise the omap4iss driver will fail to build.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>