Close and release the current usb device.
        :return: None

def close(self):
        """
        Close and release the current usb device.
        :return: None
        """
        # This may not be absolutely necessary, but it is safe.
        # It's the closest thing to a close() method.
        if self._dev is not None:
            usb.util.dispose_resources(self._dev)
            self._dev = None