device: don't allways set assumed reason when platform init is done

device: don't allways set assumed reason when platform init is done

When the device is transitioning from unmanaged to disconnected for "assumed"
reason, bring_up is not called. This is not a good thing in numerous
instances, e.g.:

1.) vlans that we've created need that to set IFF_UP and read carrier
otherwise they won't be available for connections.

2.) veths that are being managed need to start the deferred carrier check
so that the behavior matches real Ethernet.

3.) Hardware devices that were plugged in while NetworkManager is running
that need the IFF_UP for a carrier check, possibly enqueueing a deferred one.

Fixes: 5637d72af2d51b6f2b8dbe70553754f1828fdb66.