[media] uvcvideo: Generate discontinuous sequence numbers when frames are lost

Increase the sequence number of the v4l2_buffer structure regardless of
any buffer states, so that discontinuous sequence numbers allow
applications to detect lost video frames.

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