USB: fix __must_check warnings in drivers/usb/core/

Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c
index fd345ad..71bbd25 100644
--- a/drivers/usb/core/devio.c
+++ b/drivers/usb/core/devio.c
@@ -1359,7 +1359,7 @@
 	/* let kernel drivers try to (re)bind to the interface */
 	case USBDEVFS_CONNECT:
 		usb_unlock_device(ps->dev);
-		bus_rescan_devices(intf->dev.bus);
+		retval = bus_rescan_devices(intf->dev.bus);
 		usb_lock_device(ps->dev);
 		break;