V4L/DVB (13503): uvcvideo: Merge iterms, oterms and units linked lists

All terminals and units are now added to a single linked list of
entities per chain. This makes terminals and units handling code more
generic.

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