nymea-plugins-modbus pull request checklist:

- [ ] Make sure the pull request's title is of format "Plugin name: Add support for xyz" or "New plugin: Plugin name"

- [ ] Did you test the changes on hardware, if not (e.g. absence of required hardware), please mention a person to confirm it has been tested.

- [ ] Did you update the plugin's README.md accordingly?

- [ ] Did you update translations (`cd builddir && make lupdate`)?