- Moves 8 functions from usb.[hc] to hcd.[hc]
    - Also moves some data structures and types
    - Now usbdevfs and "old" HCDs #include "hcd.h"
    - Minor tweaks to the "hcd" layer (one less FIXME)
    - Minor kernel doc and comment cleanups
  
Basically this continues moving the HCD-only functionality
out of the way of normal USB device drivers.  Converging
"usb_bus" and "usb_hcd" (later!) will be a bit easier too.

I did basic sanity tests, there's little to break ... :)
  
There are still a few functions in usb.c that aren't for
general driver use.  They're mostly for enumeration,
in areas where the hub driver and HCD root hubs
need to do various kinds of magic.  It wasn't clear
how to decouple those, they can certainly wait.