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.cpp

@@ -264,7 +264,7 @@ std::string Device::GetSource() const
   return "XInput";
 }
 
-void Device::UpdateInput()
+Core::DeviceRemoval Device::UpdateInput()
 {
   PXInputGetState(m_index, &m_state_in);
 
@@ -286,6 +286,8 @@ void Device::UpdateInput()
       break;
     }
   }
+
+  return Core::DeviceRemoval::Keep;
 }
 
 void Device::UpdateMotors()