[media] uvcvideo: Ignore entities for terminals with no supported format

If a streaming interface has no supported format, the driver won't
create a video device for the associated terminal. Fix an oops by
ignoring that terminal when creating links between entities.

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