ovs: avoid race condition when system interface is removed from ovsdb
Failing the system interface device is almost always the right thing to do when the ovsdb entry is removed.
However, to avoid that a late device-removed signal tears down a different, newly-activated connection, let's also check that we have a master. Or in alternative, that the device is assumed/external: in such case it's always fine to fail the device
Fixes: 8e55efeb ('ovs: fail OVS system interfaces when the db entry gets removed')