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/WGInput/WGInput.cpp

@@ -490,7 +490,7 @@ private:
 
   std::string GetSource() const override { return std::string(SOURCE_NAME); }
 
-  void UpdateInput() override
+  Core::DeviceRemoval UpdateInput() override
   {
     // IRawGameController:
     static_assert(sizeof(bool) == sizeof(ButtonValueType));
@@ -527,6 +527,8 @@ private:
     // IGameControllerBatteryInfo:
     if (!UpdateBatteryLevel())
       DEBUG_LOG_FMT(CONTROLLERINTERFACE, "WGInput: UpdateBatteryLevel failed.");
+
+    return Core::DeviceRemoval::Keep;
   }
 
   void UpdateMotors()