USB: fix memory leak in cdc-acm

This fixes a memory leak on disconnect in cdc-acm

Thanks to 施金前 for finding it.
Signed-off-by: Oliver Neukum <oneukum@suse.de>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/class/cdc-acm.c

@@ -849,9 +849,10 @@ static void acm_write_buffers_free(struct acm *acm)
 {
 	int i;
 	struct acm_wb *wb;
+	struct usb_device *usb_dev = interface_to_usbdev(acm->control);
 
 	for (wb = &acm->wb[0], i = 0; i < ACM_NW; i++, wb++) {
-		usb_buffer_free(acm->dev, acm->writesize, wb->buf, wb->dmah);
+		usb_buffer_free(usb_dev, acm->writesize, wb->buf, wb->dmah);
 	}
 }