Plane: updates for EKF API changes

ArduPlane/ArduPlane.cpp

@@ -1016,7 +1016,7 @@ void Plane::update_optical_flow(void)
         uint8_t flowQuality = optflow.quality();
         Vector2f flowRate = optflow.flowRate();
         Vector2f bodyRate = optflow.bodyRate();
-        Vector3f posOffset = optflow.get_pos_offset();
+        const Vector3f &posOffset = optflow.get_pos_offset();
         ahrs.writeOptFlowMeas(flowQuality, flowRate, bodyRate, last_of_update, posOffset);
         Log_Write_Optflow();
     }