settings: add baud rate 230400

The Tektronix 4050 DMM supports baud rate 230400 and also the
serial port driver of the USB-to-EIA-232 cable for it supports
that baud rate. So we should support that as well. But it remains
non-standard on Windows and may require custom changes to
QextSerialPort.
DMM_send_values: fix error flow in dmmcontrol

When an error occurs, we aren't able to restart the DMM emulation
so clear the return code each iteration.