code: void onReceive(int register, byte[] message) {
          I2CEvent evt = new I2CEvent(this, register, message);
          I2CListener listener = callbacks.remove(register);
          if (listener == null) {
              for (I2CListener subscriber : subscribers) {
                  subscriber.onReceive(evt);
              }
          } else {
              listener.onReceive(evt);
          }
      }
docs: {@link FirmataDevice} calls this method when receives a message from I2C
  device.
  
  @param register The device register read from.
  @param message actual data from I2C device