[media] V4L: soc_camera_platform: do not leave dangling invalid pointers

The life-time of soc-camera device objects can be longer, than the
time, it is attached to a client driver, therefore all references to
the driver own data have to be cleared, when the driver is detached.

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