zketech-ebd-usb: Multiple fixes and upgrades.

zketech-ebd-usb: Multiple fixes and upgrades.

- fix load toggling at acquisition stop
- messages are sent byte per byte with a pause in between since bytes were
  sometimes dropped
- incoming stream is parsed continuously instead of using a fixed length (the
  messages have a constant start/stop)
- add under-voltage threshold (under which the load would stop) as configuration
- add D+/D- voltage measurements as two new channels (had to rename the
  existing to avoid confusion)