[media] soc-camera: protect against racing open(2) and rmmod

To protect against open() racing with rmmod, hold the list_lock also while
obtaining a reference to the camera host driver and check that the video
device hasn't been unregistered yet.

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