V4L/DVB: soc-camera: allow only one video queue per device

Multiple user-space application instances can open the same video device, but
it only makes sense for one of them to manage the videobuffer queue and set
video format of the device. Restrict soc-camera respectively.

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