fix for ds18b20 negative decimals
fix for ds18b20 negative decimals

ds18b20 decimals do not take into account the sign bit. Since the original calculation was not so readable, rewritten in readable way that also fixes the bug. Same code as PR against master.
vsky279/nodemcu-firmware,TerryE/nodemcu-firmware,nwf/nodemcu-firmware,FelixPe/nodemcu-firmware,eku/nodemcu-firmware,HEYAHONG/nodemcu-firmware,HEYAHONG/nodemcu-firmware,vsky279/nodemcu-firmware,marcelstoer/nodemcu-firmware,eku/nodemcu-firmware,devsaurus/nodemcu-firmware,nwf/nodemcu-firmware,devsaurus/nodemcu-firmware,vsky279/nodemcu-firmware,devsaurus/nodemcu-firmware,TerryE/nodemcu-firmware,nodemcu/nodemcu-firmware,nodemcu/nodemcu-firmware,marcelstoer/nodemcu-firmware,FelixPe/nodemcu-firmware,TerryE/nodemcu-firmware,FelixPe/nodemcu-firmware,FelixPe/nodemcu-firmware,FelixPe/nodemcu-firmware,marcelstoer/nodemcu-firmware,marcelstoer/nodemcu-firmware,eku/nodemcu-firmware,devsaurus/nodemcu-firmware,eku/nodemcu-firmware,marcelstoer/nodemcu-firmware,vsky279/nodemcu-firmware,HEYAHONG/nodemcu-firmware,HEYAHONG/nodemcu-firmware,TerryE/nodemcu-firmware,devsaurus/nodemcu-firmware,nwf/nodemcu-firmware,nwf/nodemcu-firmware,vsky279/nodemcu-firmware,nwf/nodemcu-firmware,TerryE/nodemcu-firmware,HEYAHONG/nodemcu-firmware,nodemcu/nodemcu-firmware,nodemcu/nodemcu-firmware,nodemcu/nodemcu-firmware