Close the link.

def close(self):
        """ Close the link. """
        # Stop the comm thread
        self._thread.stop()

        # Close the USB dongle
        if self._radio_manager:
            self._radio_manager.close()
        self._radio_manager = None

        while not self.out_queue.empty():
            self.out_queue.get()

        # Clear callbacks
        self.link_error_callback = None
        self.link_quality_callback = None