AP_Param: avoid a notify if value is already correct

libraries/AP_Param/AP_Param.h

@@ -536,9 +536,11 @@ public:
     /// Value setter - set value, tell GCS
     ///
     void set_and_notify(const T &v) {
+        if (v != _value) {
             set(v);
             notify();
         }
+    }
 
     /// Combined set and save
     ///