[media] media: make media_link more generic to handle interace links

By adding an union at media_link, we get for free a way to
represent interface->entity links.

No need to change anything at the code, just at the internal
header file.

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