SMPP transport is missing a yield when asking for the protocol while disconnecting.

```
2014-02-10 08:12:05+0000 [-] Unhandled Error
        Traceback (most recent call last):
          File "/mnt/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 295, in addCallbacks
            self._runCallbacks()
          File "/mnt/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 577, in _runCallbacks
            current.result = callback(current.result, *args, **kw)
          File "/mnt/praekelt/vumi-go/ve/src/vumi/vumi/transports/base.py", line 74, in <lambda>
            d.addCallback(lambda r: self.teardown_transport())
          File "/mnt/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1237, in unwindGenerator
            return _inlineCallbacks(None, gen, Deferred())
        --- <exception caught here> ---
          File "/mnt/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1099, in _inlineCallbacks
            result = g.send(result)
          File "/mnt/praekelt/vumi-go/ve/src/vumi/vumi/transports/smpp/smpp_transport.py", line 161, in teardown_transport
            yield self.service.stopService()
          File "/mnt/praekelt/vumi-go/ve/src/vumi/vumi/transports/smpp/smpp_transport.py", line 113, in stopService
            d = protocol.disconnect()
        exceptions.AttributeError: Deferred instance has no attribute 'disconnect'
```