oscplot.c: Fix validation method for devices with trigger
Allow capture to be started when a trigger device has no trigger associated provided that the all channels of the device are not enabled.
plugins/fmcomms2.c: Use 8 scales (fmcomms2), 4 scales (fmcomms4)
plugins/fmcomms2.c: Fix bug when making dds scales signal-based
When pairing two dds scales to check if their sum exceeds 1, make sure that we start couting the widgets from 0