Plane: reset land_pre_flare on mode change

ArduPlane/system.cpp

@@ -342,6 +342,9 @@ void Plane::set_mode(enum FlightMode mode)
     // reset go around command
     auto_state.commanded_go_around = false;
 
+    // not in pre-flare
+    auto_state.land_pre_flare = false;
+    
     // zero locked course
     steer_state.locked_course_err = 0;