[media] sh-mobile-ceu-camera: move interface activation and deactivation to clock callbacks

When adding and removing a client, the sh-mobile-ceu-camera driver activates
and, respectively, deactivates its camera interface and, if necessary, the
CSI2 controller. Only handling of the CSI2 interface is client-specific and
is only needed, when a data-exchange with the client is taking place. Move
the rest to .clock_start() and .clock_stop() callbacks.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>