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

Don't handle the first output terminal in a chain in a special way. Use
uvc_scan_chain_entity() like for all other entities, making the chain
scan code more generic.

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