INA220 readings sometimes report negative current

The INA220 sensor sometimes report a negative current on the FMC power channels, especially when they are disconnected and/or disabled.
This could be a data format error, or even a inverted measure pin on the shunt resistor.

Start by locating the INA220 handling and FMC power-channel current conversion in the C firmware. Reproduce readings with channels disconnected or disabled, then trace whether the result comes from data formatting or shunt measurement polarity; done means the cause is confirmed and the reported current behaves correctly in those cases.

Written by the indexing model from the issue text.