[media] v4l: vsp1: Check if an entity is a subdev with the right function

Use is_media_entity_v4l2_subdev() instead of is_media_entity_v4l2_io()
to check whether the entity is a subdev.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>