Use CRSF "Bat" sensor as fuel source when available

[jp39]

With CRSF, INAV reports the remaining battery percentage as the "Bat" sensor. This sensor already takes into account the battery capacity from INAV, if configured properly, using the same calculateBatteryPercentage function as smartport telemetry when smartport_fuel_unit=PERCENT.

Instead of having to enter the battery capacity in both the widget settings and INAV, allow INAV to be the primary source of truth for the fuel information.