usbip: move usbip_protocol.txt to Documentation

The usbip_protocol.txt, a document which describes usbip's
inner workings is currently located in the projects source
directory (drivers/usb/usbip/...). This patch moves it to
Documentation/usb.

This discussion was brought up by Guy Harris [0] during the
review of the USBIP dissector I wrote. For anyone interested:
support is available with the latest wireshark master/dev tree.
Simply select a packet from the usbip's tcp-stream you are
intrested on and select the USBIP as the protocol in the
"Decode As" dialog box [1].

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>

[0] <https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=12127#c2>
[1] <https://www.wireshark.org/docs/wsug_html_chunked/ChCustProtocolDissectionSection.html#ChAdvDecodeAs

>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>