[media] media: make add link more generic

The media_entity_add_link() function takes an entity
as an argument just to get the list head.

Make it more generic by changing the function argument
to list_head.

No functional changes.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>