Plane: Address Randy review comments

ArduPlane/Attitude.cpp

@@ -54,13 +54,16 @@ float Plane::get_speed_scaler(void)
  */
 bool Plane::stick_mixing_enabled(void)
 {
+#if AC_FENCE == ENABLED
+    const bool stickmixing = fence_stickmixing();
+#else
+    const bool stickmixing = true;
+#endif
     if (control_mode->does_auto_throttle() && plane.control_mode->does_auto_navigation()) {
         // we're in an auto mode. Check the stick mixing flag
         if (g.stick_mixing != STICK_MIXING_DISABLED &&
             g.stick_mixing != STICK_MIXING_VTOL_YAW &&
-            #if AC_FENCE == ENABLED
-            fence_stickmixing() &&
-            #endif
+            stickmixing &&
             failsafe.state == FAILSAFE_NONE &&
             !rc_failsafe_active()) {
             // we're in an auto mode, and haven't triggered failsafe

ArduPlane/radio.cpp

@@ -186,13 +186,16 @@ void Plane::read_radio()
 
     control_failsafe();
 
+#if AC_FENCE == ENABLED
+    const bool stickmixing = fence_stickmixing();
+#else
+    const bool stickmixing = true;
+#endif
     airspeed_nudge_cm = 0;
     throttle_nudge = 0;
-    if (g.throttle_nudge && channel_throttle->get_control_in() > 50
-        #if AC_FENCE == ENABLED
-        && fence_stickmixing()
-        #endif
-        ) {
+    if (g.throttle_nudge
+        && channel_throttle->get_control_in() > 50
+        && stickmixing) {
         float nudge = (channel_throttle->get_control_in() - 50) * 0.02f;
         if (ahrs.airspeed_sensor_enabled()) {
             airspeed_nudge_cm = (aparm.airspeed_max * 100 - aparm.airspeed_cruise_cm) * nudge;