usb.rst: move documentation from proc_usb_info.txt to USB ReST book

The contents of proc_usb_info.txt complements what's there at
driver-api usb book. Yet, it is outdated, as it still refers
to the USB character devices as usbfs.

So, move the contents to usb.rst, adjusting it to point to
the right places.
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>