media: uvcvideo: Convert from using an atomic variable to a reference count

When adding support for metadata nodes, we'll have to keep video
devices registered until all metadata nodes are closed too. Since
this has nothing to do with stream counting, replace the nstreams
atomic variable with a reference counter.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>