Fix usb scan delaying shutdown (#116390)

If the integration page is accessed right before shutdown it
can trigger the usb scan debouncer which was not marked as
background so shutdown would wait for the scan to finish

homeassistant/components/usb/__init__.py

@@ -394,6 +394,7 @@ class USBDiscovery:
                 cooldown=REQUEST_SCAN_COOLDOWN,
                 immediate=True,
                 function=self._async_scan,
+                background=True,
             )
         await self._request_debouncer.async_call()