usb/callbacks.txt: convert to ReST and add to driver-api book

This document describe some USB core functions. Add it to the
driver-api book.

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>

Documentation/driver-api/usb/index.rst

@@ -8,6 +8,7 @@ Linux USB API
    gadget
    anchors
    bulk-streams
+   callbacks
    writing_usb_driver
    writing_musb_glue_layer