[media] uvcvideo: Tell the user space we're using start-of-exposure timestamps

The UVC device provided timestamps are taken from the clock once the
exposure of the frame has begun, not when the reception of the frame would
have been finished as almost anywhere else. Show this to the user space by
using V4L2_BUF_FLAG_TSTAMP_SRC_SOE buffer flag.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>