Documentation/driver-api/usb: Replace dead link

This link is dead:

    $ curl -vI http://usb.cs.tum.edu/usbdoc


    * Could not resolve host: usb.cs.tum.edu
    * Closing connection 0
    curl: (6) Could not resolve host: usb.cs.tum.edu

I found the document somewhere else. Let's replace it.

Signed-off-by: Stefan Tatschner <stefan.tatschner@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>