usb: gadget: fix the mtp failure in win7

Change-Id: I9a8274cd81f7cb9d3aa48a6fd6d35fcc06cab071
diff --git a/drivers/usb/gadget/f_mtp.c b/drivers/usb/gadget/f_mtp.c
index 0394b0b..a7c728d 100644
--- a/drivers/usb/gadget/f_mtp.c
+++ b/drivers/usb/gadget/f_mtp.c
@@ -1098,6 +1098,7 @@
 	if (id < 0)
 		return id;
 	mtp_interface_desc.bInterfaceNumber = id;
+	mtp_ext_config_desc.function.bFirstInterfaceNumber = id;
 
 	/* allocate endpoints */
 	ret = mtp_create_bulk_endpoints(dev, &mtp_fullspeed_in_desc,