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/XInput/XInput.h

@@ -33,7 +33,7 @@ public:
   std::optional<int> GetPreferredId() const override;
   int GetSortPriority() const override { return -2; }
 
-  void UpdateInput() override;
+  Core::DeviceRemoval UpdateInput() override;
 
   void UpdateMotors();