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/evdev/evdev.h

@@ -72,7 +72,7 @@ private:
   };
 
 public:
-  void UpdateInput() override;
+  Core::DeviceRemoval UpdateInput() override;
   bool IsValid() const override;
 
   evdevDevice(InputBackend* input_backend);