V4L/DVB (13132): fix use-after-free Oops, resulting from a driver-core API change

Commit b4028437 has broken again re-use of
device objects across device_register() / device_unregister() cycles. Fix
soc-camera by nullifying the struct after device_unregister().

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