[Bug] Issue with "Consumption" and negative values when used for PV-measurement
**Describe the bug**
I am using a Shelly 1 PM with my PV-Panels to measure how much energy I can produce.
The Shelly App is showing these as positive values, however, since I am not drawing power but feed in power, the value is negative.
It shows fine as positive in Homekit, but the Plugin creates tons of Log entries like the one below

**Log output**
[03/03/2021, 08:39:51] [homebridge-shelly] This plugin generated a warning from the characteristic 'Consumption': characteristic was supplied illegal value: number -57.17 exceeded minimum of 0. See https://git.io/JtMGR for more info.

**Environment:**
 - Shelly device(s): Shelly 1 PM
 - Plugin version: 0.18.0
 - Device running homebridge: Raspberry Pi 3
