[media] pxa-camera: move interface activation and deactivation to clock callbacks

When adding and removing a client, the pxa-camera driver only activates
and deactivates its camera interface respectively, which doesn't include
any client-specific actions. Move this functionality into .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>