Fix calculating brightness percentage of 0% causing bad request

[nicodreher]

Add lower limit for brightness to 1%.

fixes #208

[tschamm]

Which kind of light type you have testet that? I tested the current version with Hue White Ambiance lights. HA has a brightness range from 0...255, while Bosch supports brightness values from 0..100.
When setting brightness to 1%, everything is ok. When setting brightness to 1 in HA, the light turns off, as the conversion leads to an api call with brightness 0.

[nicodreher]

I have tested it with the Bosch Smart Home Dimmer https://www.bosch-smarthome.com/de/de/produkte/geraete/dimmer/

Setting the Brightness to 1% works fine. But setting it to 1 in HA leads the following error: SHC Error (message: API call returned non-OK result (code 400)!: b'{"@type":"JsonRestExceptionResponseEntity","errorCode":"INVALID_REQUEST_OBJECT","statusCode":400}')

[tschamm]

Thank you, I merged your changes.