Transport bled112 intermittent failure to stop scan

Every so often when using multiple BLED112 dongles, there is an intermittent issue that shows up when connecting. It is unknown what causes this.

```
[POD1MFullProcessorActor-6] Device found and connectable: d--0000-0000-0000-0a4a
[POD1MFullProcessorActor-6] Using BLED112 adapter at /dev/cu.usbmodem11
[POD1MFullProcessorActor-6] BLED112 adapter supports 3 connections
[EventLoopThread] Stopping adapter wrapper
[EventLoopThread] Error running finalizer for task LegacyAdapter (BLED112Adapter)
Traceback (most recent call last):
File "/Users/lukee/arch/env/py38/lib/python3.8/site-packages/iotile/core/utilities/async_tools/event_loop.py", line 195, in stop
await result
File "/Users/lukee/arch/env/py38/lib/python3.8/site-packages/iotile/core/hw/transport/adapter/async_wrapper.py", line 106, in _stop_internal
await self._execute(self._adapter.stop_sync)
File "/usr/local/Cellar/python@3.8/3.8.1/Frameworks/Python.framework/Versions/3.8/lib/python3.8/concurrent/futures/thread.py", line 57, in run
result = self.fn(*self.args, **self.kwargs)
File "/Users/lukee/arch/env/py38/lib/python3.8/site-packages/iotile_transport_bled112/bled112.py", line 195, in stop_sync
self.stop_scan()
File "/Users/lukee/arch/env/py38/lib/python3.8/site-packages/iotile_transport_bled112/bled112.py", line 211, in stop_scan
self._command_task.sync_command(['_stop_scan'])
File "/Users/lukee/arch/env/py38/lib/python3.8/site-packages/iotile_transport_bled112/bled112_cmd.py", line 806, in sync_command
raise HardwareError("Error executing synchronous command", command=cmd, return_value=retval)
iotile.core.exceptions.HardwareError: HardwareError: Error executing synchronous command
Additional Information:
command: ['_stop_scan']
return_value: {'reason': 'Could not stop scan for ble devices'}
[EventLoopThread] Stopping adapter wrapper
```