iface-modem-simple: fix Simple.Disconnect() when bearer path given

The Simple.Disconnect() method handler was totally broken when a specific bearer path was given, because the user-provided bearer path was never being stored in the DisconnectionContext struct...

This is an extremely bad error, but it also gives us an indication that no one is really using this method to disconnect one single bearer, mainly because we also have the Bearer.Disconnect() method, which makes more sense. mmcli didn't even allow passing a bearer path to --simple-disconnect.