networkd: link - don't enforce ENSLAVING state

We are only guaranteed to stay in ENSLAVING state whilst enslaving by bridges/bonds, not
when adding stacked devices (as then the underlying device can be IFF_UP'ed and configured
in parallel), so drop these asserts.
networkd: netdev - add missing callback when adding stacked devices

As the comment says, the passed in callback must always be invoked, or the underlying link
will hang. This was missed when reworking the code, so add it back in.