[media] uvcvideo: Don't record timespec_sub

The statistics function subtracts two timespecs manually. A helper is
provided by the kernel to do this.

Replace the implementation, using the helper.

Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>