[media] [FOR,v3.3] uvcvideo: Avoid division by 0 in timestamp calculation

Use system timestamps if the timestamp can't be computed from the data
sent by the device.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>