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: NKieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>