Disconnecting a bled112 dongle, but leaving your program on causes high CPU usage

Steps to reproduce:

Start iotile hw --port=bled112

(run scan, or something that uses the transport plugin)

Unplug your bled112 dongle

Watch activity monitor.

If you let it go long enough, you'll get errors like this when you close your program eventually:
```

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/local/Cellar/python35/3.5.6_2/Frameworks/Python.framework/Versions/3.5/lib/python3.5/threading.py", line 914, in _bootstrap_inner
self.run()
File "/Users/mrunchey/v.8/lib/python3.5/site-packages/iotile_transport_bled112/bled112_cmd.py", line 48, in run
res = getattr(self, cmd)(*args)
File "/Users/mrunchey/v.8/lib/python3.5/site-packages/iotile_transport_bled112/bled112_cmd.py", line 662, in _disconnect
response = self._send_command(3, 0, payload)
File "/Users/mrunchey/v.8/lib/python3.5/site-packages/iotile_transport_bled112/bled112_cmd.py", line 700, in _send_command
self._stream.write(bytes(packet))
File "/Users/mrunchey/v.8/lib/python3.5/site-packages/iotile_transport_bled112/async_packet.py", line 34, in write
raise DeviceNotConfiguredError("Device not configured properly")
iotile_transport_bled112.async_packet.DeviceNotConfiguredError: Device not configured properly

```