[media] soc-camera: move common code to soc_camera.c

All soc-camera host drivers include a pointer to an soc-camera device in
their host private struct to check, that only one client is connected.
Move this common code to soc_camera.c.

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>