docs-rst: fix usb cross-references

As some USB documentation files got moved, adjust their
cross-references to their new place.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/driver-api/usb/persist.rst b/Documentation/driver-api/usb/persist.rst
index ea1b43f..08cafc6 100644
--- a/Documentation/driver-api/usb/persist.rst
+++ b/Documentation/driver-api/usb/persist.rst
@@ -1,3 +1,5 @@
+.. _usb-persist:
+
 USB device persistence during system suspend
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~