ArduPlane: invalidate param count when changing param tree structure

ArduPlane/quadplane.cpp

@@ -754,6 +754,9 @@ bool QuadPlane::setup(void)
 
     AP_Param::convert_old_parameters(&q_conversion_table[0], ARRAY_SIZE(q_conversion_table));
 
+    // param count will have changed
+    AP_Param::invalidate_count();
+
     gcs().send_text(MAV_SEVERITY_INFO, "QuadPlane initialised");
     initialised = true;
     return true;