media: uvcvideo: Factor out video device registration to a function

The function will then be used to register the video device for metadata
capture.

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