Input: Improve Controller Interface devices threading

This specific issue was already addressed by https://github.com/dolphin-emu/dolphin/pull/11635
though I felt like there was something more we could do, and wasn't too happy with the
likelihood of devices update calls being skipped (due to `m_devices_population_mutex` being locked).

Source/Core/InputCommon/ControllerInterface/DInput/DInputKeyboardMouse.h

@@ -94,7 +94,7 @@ private:
   };
 
 public:
-  void UpdateInput() override;
+  Core::DeviceRemoval UpdateInput() override;
 
   KeyboardMouse(const LPDIRECTINPUTDEVICE8 kb_device, const LPDIRECTINPUTDEVICE8 mo_device);
   ~KeyboardMouse();