flightaxis SITL - incorrect current reading when negative values

#### Issue details
When I'm using SITL with RF9 and flighaxis controller, negative current values in RF9 (when motor is braking) gets false high readings in Mission Planner. This causes false drop in battery percentage. It seems like an issue with signs and flightaxis/MP link.

RF9 current reading (-1.1A):

Mission Planner current reading (327,7A):

#### Version
1.3.74.1

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ X ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
plane, quadplane

#### Hardware type
SITL / flightaxis

Reproduce the issue with Plane or quadplane SITL using the FlightAxis controller and an RF9 negative current reading. Trace that value into Mission Planner and verify that negative readings are preserved rather than displayed as approximately 327.7 A, with no resulting false battery-percentage drop.

Written by the indexing model from the issue text.