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.cpp

@@ -674,7 +674,7 @@ void InputBackend::RemoveDevnodeObject(const std::string& node)
   m_devnode_objects.erase(node);
 }
 
-void evdevDevice::UpdateInput()
+Core::DeviceRemoval evdevDevice::UpdateInput()
 {
   // Run through all evdev events
   // libevdev will keep track of the actual controller state internally which can be queried
@@ -691,6 +691,7 @@ void evdevDevice::UpdateInput()
         rc = libevdev_next_event(node.device, LIBEVDEV_READ_FLAG_NORMAL, &ev);
     }
   }
+  return Core::DeviceRemoval::Keep;
 }
 
 bool evdevDevice::IsValid() const