[media] media-entity: add a helper function to create interface

As we'll be adding other interface types in the future, put the
common interface create code on a separate function.

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