AP_NavEKF : Change mag fusion to use corrected compass values

This enables compass units to be switched in-flight

libraries/AP_NavEKF/AP_NavEKF.cpp

@@ -3922,8 +3922,7 @@ void NavEKF::readMagData()
         // store time of last measurement update
         lastMagUpdate = _ahrs->get_compass()->last_update;
 
-        // Read compass data
+        // read compass data and scale to improve numerical conditioning
-        // We scale compass data to improve numerical conditioning
         magData = _ahrs->get_compass()->get_field() * 0.001f;
 
         // get states stored at time closest to measurement time after allowance for measurement delay