AP_NavEKF3: fixed init of earth state mag variances

we were starting with zero variance for X and Y earth states, it
should start with sq(magNoise)

libraries/AP_NavEKF3/AP_NavEKF3_Control.cpp

@@ -121,7 +121,7 @@ void NavEKF3_core::setWindMagStateLearningMode()
             P[21][21] = bodyMagFieldVar.z;
         } else {
             // set the variances equal to the observation variances
-            for (uint8_t index=18; index<=21; index++) {
+            for (uint8_t index=16; index<=21; index++) {
                 P[index][index] = sq(frontend->_magNoise);
             }