Close the connection to the easyfire unit.

def _close_connection(self):
        """Close the connection to the easyfire unit."""
        if (self._mode == PROP_MODE_SERIAL):
            self._serial.close()
        elif (self._mode == PROP_MODE_TCP):
            self._socket.close()
        elif (self._mode == PROP_MODE_FILE):
            self._file.close()