[media] media: move MEDIA_LNK_FL_INTERFACE_LINK logic to link creation

Instead of flagging an interface link as MEDIA_LNK_FL_INTERFACE_LINK
only when returning to userspace, do it at link creation time.

That would allow using such flag internally, and cleans up a
little bit the code for G_TOPOLOGY ioctl.

[mchehab@osg.samsung.com: folded with a fixup from Dan Carpenter,
 replacing & by &&]
Suggested-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>