QModbusTcpClient never close on disconnect

If you call the disconnectDevice() method on a QModbusTcpClient object that was not opened or has already been closed, then we would never close and would always be in QModbusDevice::ClosingState.

Status QModbusDevice::ClosingState change to QModbusDevice:: UnconnectedState only when the disconnected signal comes from the internal TcpSocket (see qmodbustcpclient_p.h), but when the socket was not open or has already been closed, signal disconnected is not emitted!

Source code to illustrate:

qmodbusdevice.cpp

void QModbusDevice::disconnectDevice()
{
    setState(QModbusDevice::ClosingState);

    //Unconnected is set by backend -> might be delayed by event loop
    close();
}

qmodbustcpclient.cpp

void QModbusTcpClient::close()
{
    if (state() == QModbusDevice::UnconnectedState)
        return;

    Q_D(QModbusTcpClient);
    d->m_socket->disconnectFromHost();
}

I think we should add an additional check to the method void QModbusTcpClient::close() for reset ClosingState to UnconnectedState