V4L/DVB (12536): soc-camera: remove .gain and .exposure struct soc_camera_device members

This makes the soc-camera interface for V4L2 subdevices thinner yet. Handle
gain and exposure internally in each driver just like all other controls.

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