Send STOP setpoing, stopping the motors and (potentially) falling.

def send_stop_setpoint(self):
        """
        Send STOP setpoing, stopping the motors and (potentially) falling.
        """
        pk = CRTPPacket()
        pk.port = CRTPPort.COMMANDER_GENERIC
        pk.data = struct.pack('<B', TYPE_STOP)
        self._cf.send_packet(pk)