hameg-hmo: Try to find a valid serialcomm if none is supplied.

hameg-hmo: Try to find a valid serialcomm if none is supplied.

If no serial port option is specified on the command-line using the
"serialcomm" driver option, but the device is connected through USB
and it requires a known default serial port option, then use it in
order to avoid data corruption or even worse problems.

Note: the easiest way to reproduce data corruption on HMO3000 series
is to start an analog data acquisition (e.g. on channel CH1) after
switching from Ethernet mode to USB VCP mode (HO732 USB/Ethernet interface).

This fixes parts of bug #1321.