lx_emul: fix unplug of unclaimed USB devices

USB devices that are never associated to client sessions lack
usb_per_dev_data (and a kernel thread). Therefore, the devices should be
discontinued directly on unplug as no URBs can be pending.

Issue genodelabs/genode#4795

repos/dde_linux/src/lib/lx_emul/usb.c

@@ -800,6 +800,9 @@ static int raw_notify(struct notifier_block *nb, unsigned long action, void *dat
 			if (data) {
 				data->dev = NULL;
 				lx_emul_task_unblock(data->task);
+			} else {
+				/* discontinue unclaimed device */
+				genode_usb_discontinue_device(udev->bus->busnum, udev->devnum);
 			}
 			break;
 		}