Regression in QSerialPort since 5.10

 #if defined(Q_OS_WIN32)
    // We need try to start async reading to read a remainder from a driver's queue
    // in case we have a limited read buffer size. Because the read notification can
    // be stalled since Windows do not re-triggered an EV_RXCHAR event if a driver's
    // buffer has a remainder of data ready to read until a new data will be received.
    Q_D(QSerialPort);
    if (d->readBufferMaxSize || d->flowControl == QSerialPort::HardwareControl)
        d->startAsyncRead();
#elif defined(Q_OS_UNIX)
    // We need try to re-trigger the read notification to read a remainder from a
    // driver's queue in case we have a limited read buffer size.
    Q_D(QSerialPort);
    if (d->readBufferMaxSize && !d->isReadNotificationEnabled())
        d->setReadNotificationEnabled(true);
#endif