usb: gadget: f_rndis: fix interface id for OS descriptors

f->os_desc_table[0].if_id is zero by default. If the actual id happens
to be different then no Feature Descriptors will be returned to the host
for this interface, so assign if_id as soon as it is known.

Cc: <stable@vger.kernel.org> # v3.16
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/function/f_rndis.c

@@ -727,6 +727,10 @@ rndis_bind(struct usb_configuration *c, struct usb_function *f)
 	rndis_control_intf.bInterfaceNumber = status;
 	rndis_union_desc.bMasterInterface0 = status;
 
+	if (cdev->use_os_string)
+		f->os_desc_table[0].if_id =
+			rndis_iad_descriptor.bFirstInterface;
+
 	status = usb_interface_id(c, f);
 	if (status < 0)
 		goto fail;