libusb: lookup devfs/ instead of /dev/ when connecting

`/dev/` does not exist when the usb is loaded before the fs (e.g. when
umass is used for rootfs mounting)

JIRA: RTOS-937

libusb/driver.c

@@ -29,7 +29,7 @@ int usb_connect(const usb_device_id_t *filters, int nfilters, unsigned drvport)
 	usb_msg_t *umsg = (usb_msg_t *)&msg.i.raw;
 	oid_t oid;
 
-	while (lookup("/dev/usb", NULL, &oid) < 0)
+	while (lookup("devfs/usb", NULL, &oid) < 0)
 		usleep(1000000);
 
 	msg.type = mtDevCtl;