ControllerInterface: invoke callbacks in AddDevice/RemoveDevice

Some backends already cause this to happen, so make it consistent across
systems.

Source/Core/InputCommon/ControllerInterface/ControllerInterface.h

@@ -54,9 +54,10 @@ public:
   void UpdateInput();
 
   void RegisterHotplugCallback(std::function<void(void)> callback);
-  void InvokeHotplugCallbacks() const;
 
 private:
+  void InvokeHotplugCallbacks() const;
+
   std::vector<std::function<void()>> m_hotplug_callbacks;
   bool m_is_init;
   void* m_hwnd;