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: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>