[media] V4L: soc-camera: camera client operations no longer compulsory

With the transition of all soc-camera host drivers to use V4L2
subdevice .[gs]_mbus_config() operations, soc-camera client operations
no longer have to be compulsory.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>