usb: gadget: g_dnl: only set iSerialNumber if we have a serial#

We don't want to claim that we support a serial number string and
later return nothing. Because of that, if g_dnl_serial is an empty
string, let's skip setting iSerialNumber to a valid number.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/g_dnl.c

@@ -224,12 +224,14 @@ static int g_dnl_bind(struct usb_composite_dev *cdev)
 	g_dnl_string_defs[1].id = id;
 	device_desc.iProduct = id;
 
-	id = usb_string_id(cdev);
-	if (id < 0)
-		return id;
+	if (strlen(g_dnl_serial)) {
+		id = usb_string_id(cdev);
+		if (id < 0)
+			return id;
 
-	g_dnl_string_defs[2].id = id;
-	device_desc.iSerialNumber = id;
+		g_dnl_string_defs[2].id = id;
+		device_desc.iSerialNumber = id;
+	}
 
 	g_dnl_bind_fixup(&device_desc, cdev->driver->name);
 	ret = g_dnl_config_register(cdev);