Plane: fixed loiter mode when Q_GUIDED_MODE=1

This fixes the fixed wing behaviour of LOITER mode whne
Q_GUIDED_MODE=1.

Many thanks to the PertUAV team for finding and reporting this bug

ArduPlane/navigation.cpp

@@ -173,8 +173,7 @@ void Plane::update_loiter(uint16_t radius)
     }
 
     if (loiter.start_time_ms != 0 &&
-        quadplane.available() &&
-        quadplane.guided_mode != 0) {
+        quadplane.guided_mode_enabled()) {
         if (!auto_state.vtol_loiter) {
             auto_state.vtol_loiter = true;
             // reset loiter start time, so we don't consider the point
@@ -202,8 +201,7 @@ void Plane::update_loiter(uint16_t radius)
                 // starting a loiter in GUIDED means we just reached the target point
                 gcs_send_mission_item_reached_message(0);
             }
-            if (quadplane.available() &&
-                quadplane.guided_mode != 0) {
+            if (quadplane.guided_mode_enabled()) {
                 quadplane.guided_start();
             }
         }

ArduPlane/quadplane.cpp

@@ -2037,3 +2037,18 @@ void QuadPlane::afs_terminate(void)
         motors->output();
     }
 }
+
+/*
+  return true if we should do guided mode loitering using VTOL motors
+ */
+bool QuadPlane::guided_mode_enabled(void)
+{
+    if (!available()) {
+        return false;
+    }
+    // only use quadplane guided when in AUTO or GUIDED mode
+    if (plane.control_mode != GUIDED && plane.control_mode != AUTO) {
+        return false;
+    }
+    return guided_mode != 0;
+}

ArduPlane/quadplane.h

@@ -347,6 +347,7 @@ private:
     void tilt_compensate(float *thrust, uint8_t num_motors);
 
     void afs_terminate(void);
+    bool guided_mode_enabled(void);
     
 public:
     void motor_test_output();