[media] go7007: fix unregister/disconnect handling

- use the v4l2_device's release() callback
- remove the unnecessary ref_count
- don't free usb data structures on disconnect, only do that in the final
  release callback.
This is the correct way in order to safely handle disconnect and removal
of modules.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>