USB: add read support to usb-serial/../new_id

Keep the usb-serial support for dynamic IDs in sync with the usb
support.  This enables readout of dynamic device IDs for
usb-serial drivers.  Common code is exported from the usb core
system and reused by the usb-serial bus driver.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/include/linux/usb.h b/include/linux/usb.h
index 5483cd7..1493345 100644
--- a/include/linux/usb.h
+++ b/include/linux/usb.h
@@ -790,6 +790,8 @@
 				struct device_driver *driver,
 				const char *buf, size_t count);
 
+extern ssize_t usb_show_dynids(struct usb_dynids *dynids, char *buf);
+
 /**
  * struct usbdrv_wrap - wrapper for driver-model structure
  * @driver: The driver-model core driver structure.