[media] v4l: subdev: Don't require core operations

There's no reason to require subdevices to implement the core
operations. Remove the check for non-NULL core operations when
initializing the subdev.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>