update battery min max value to better recreate app expierience

[Robert-Riedl]

Hi, so i've wondered for a couple of days why my waveplus has "0%" battery but is still reporting its other values accurately.

So i've checked with the offical app and to my surprise the battery value there was 3% - and the other waveplus devices all had 100%, while my mqtt values were in the 20s and 30s.

So I've dug around in your code and found that you've defined rather high values for bat_min and bat_max.

I've changed them to 2.1 and 2.4 respectivly - tell me what you think.