Fix "lux" warning

[Crayjin]

Fixes invalid value usage of native unit of measurement "lux" warning by using const enums.

[Crayjin]

Unfortunately, this will lead to other warnings

• The unit of sensor.elv_weather_station_light is changing, got multiple {'lux', 'lx'}, generation of long term statistics will be suppressed unless the unit is stable and matches the unit of already compiled statistics (lux). Go to https://my.home-assistant.io/redirect/developer_statistics to fix this

• The unit of sensor.elv_weather_station_uv is changing, got multiple {'W/m2', 'W/m²'}, generation of long term statistics will be suppressed unless the unit is stable. Go to https://my.home-assistant.io/redirect/developer_statistics to fix this

First warning can be fixed via http://hass.instance/developer-tools/statistics. for the second warning HASS UI doesn't provide an fix action.

[carstenschroeder]

@Crayjin Thx!