AP_NavEKF2: make mag variance reporting consistent

logged scaled variance should match the value used in MAVLink
EKF_STATUS_REPORT

libraries/AP_NavEKF2/AP_NavEKF2_Outputs.cpp

@@ -578,6 +578,8 @@ void NavEKF2_core::send_status_report(mavlink_channel_t chan)
     Vector2f offset;
     getVariances(velVar, posVar, hgtVar, magVar, tasVar, offset);
 
+    const float mag_max = fmaxf(fmaxf(magVar.x,magVar.y),magVar.z);
+
     // Only report range finder normalised innovation levels if the EKF needs the data for primary
     // height estimation or optical flow operation. This prevents false alarms at the GCS if a
     // range finder is fitted for other applications
@@ -589,7 +591,7 @@ void NavEKF2_core::send_status_report(mavlink_channel_t chan)
     }
 
     // send message
-    mavlink_msg_ekf_status_report_send(chan, flags, velVar, posVar, hgtVar, magVar.length(), temp, tasVar);
+    mavlink_msg_ekf_status_report_send(chan, flags, velVar, posVar, hgtVar, mag_max, temp, tasVar);
 }
 
 // report the reason for why the backend is refusing to initialise