AP_Frsky_Telem: actually set _ap.value flags based in is_flying

Closes #5017

libraries/AP_Frsky_Telem/AP_Frsky_Telem.cpp

@@ -907,3 +907,12 @@ void AP_Frsky_Telem::calc_gps_position(void)
         _gps.speed_in_centimeter = 0;
     }
 }
+
+void AP_Frsky_Telem::set_is_flying(bool is_flying)
+{
+    if (is_flying) {
+        _ap.value |= AP_ISFLYING_FLAG;
+    } else {
+        _ap.value &= ~AP_ISFLYING_FLAG;
+    }
+}

libraries/AP_Frsky_Telem/AP_Frsky_Telem.h

@@ -127,7 +127,7 @@ public:
     void update_control_mode(uint8_t mode) { _ap.control_mode = mode; }
 
     // update whether we're flying (used for Plane)
-    void set_is_flying(bool is_flying) { is_flying ? (_ap.value | AP_ISFLYING_FLAG) : (_ap.value & (~AP_ISFLYING_FLAG)); }
+    void set_is_flying(bool is_flying);
  
     // update error mask of sensors and subsystems. The mask uses the
     // MAV_SYS_STATUS_* values from mavlink. If a bit is set then it