att pos estimator: correctly initialize fusionModeGPS variable, this makes the system use gps again and hence fixes the huge drift problems

src/modules/ekf_att_pos_estimator/estimator_23states.cpp

@@ -14,6 +14,7 @@ AttPosEKF::AttPosEKF()
     summedDelAng.zero();
     summedDelVel.zero();
 
+    fusionModeGPS = 0;
     fuseVelData = false;
     fusePosData = false;
     fuseHgtData = false;