[media] uvcvideo: Fix descriptor parsing for video output devices

Commit 4057ac6c

    V4L/DVB (13505): uvcvideo: Refactor chain scan

broke output terminals parsing. Fix it.

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