[media] soc_camera: replace vdev->parent by vdev->v4l2_dev

The parent field will eventually disappear to be replaced by v4l2_dev.
soc_camera does provide a v4l2_device struct but did not point to it in
struct video_device. This is now fixed.
Now the video nodes can be found under the correct platform bus, and
the advanced debug ioctls work correctly as well (the core implementation
of those ioctls requires that v4l2_dev is set correctly).

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>