Fix missing device_class from 2021.12.1 on

[weltenwort]

📝 Summary

This moves the device_class down the RctPowerSensorEntity inheritance hierarchy to adapt to the fact that the SensorEntity now overrides the device_class property of the base Entity.

closes #170

🕵️‍♀️ Notes

When the integration was installed in a Home Assistant instance for the first time before this fix but after upgrading, the recorder might have incorrectly recorded statistics using W and Wh instead of the kW and kWh expected by the Energy dashboard. In order for the entities to be selectable in the Energy configuration again, it might be necessary to use the "Fix it" feature of the "Statistics" tab in the developer tools.

❤️ Attribution

Thanks to @pluskal for first reporting it and to @Benjamin410 for assisting in the investigation.

[github-actions]

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