Plane: only revert a tuning parameter if it has been set

fixes a 2nd transmitter tuning bug found by expntly

ArduPlane/tuning.cpp

@@ -275,7 +275,11 @@ void AP_Tuning_Plane::reload_value(uint8_t parm)
     default:
         AP_Float *f = get_param_pointer(parm);
         if (f != nullptr) {
-            f->load();
+            uint64_t param_bit = (1ULL << parm);
+            // only reload if we have set this parameter at some point
+            if (param_bit & have_set) {
+                f->load();
+            }
         }
         break;
     }