ofproto: Adjust netdev_monitor when switching netdevs.

This fixes a segfault in the "ofproto - mod-port" test.  The segfault
should not occur--there must be a bug in the netdev_monitor or possibly
the netdev_dummy implementation--but the netdev_monitor_remove() and
netdev_monitor_add() calls are definitely wanted here in any case to ensure
that the new netdev, not the old one, is what gets monitored.