[media] uvcvideo: (cosmetic) Add and use an inline function

Add an inline function to obtain a struct uvc_buffer pointer from a
struct vb2_v4l2_buffer one.

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