usb: gadget: uvc: Switch to monotonic clock for buffer timestamps

The wall time clock isn't useful for applications as it can jump around
due to time adjustement. Switch to the monotonic clock.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>