[media] uvcvideo: Fix crash when linking entities

The uvc_mc_register_entity() function wrongfully selects the
media_entity associated with a UVC entity when creating links. This
results in access to uninitialized media_entity structures and can hit a
BUG_ON statement in media_entity_create_link(). Fix it.

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