Another UHCI patch.  I'm sending this since Dan said he was going to
start teaching "uhci-hcd" how to do control and interrupt queueing,
and this may help.  Granted it checks out (I didn't test the part
that has a chance to break, though it "looks right"), I think it
should get merged in at some point.  What it does:

   - updates and adds some comments/docs
   - gets rid of a "magic number" calling convention, instead passing
     an explicit flag UHCI_PTR_DEPTH or UHCI_PTR_BREADTH (self-doc :)
   - deletes bits of unused/dead code
   - updates the append-to-qh code:
       * start using list_for_each() ... clearer than handcrafted
         loops, and it prefetches too.  Lots of places should get
         updated to do this, IMO.
       * re-orders some stuff to fix a sequencing problem
       * adds ascii-art to show how the urb queueing is done
         (based on some email Johannes sent me recently)

That sequencing problem is that when splicing a QH between A and B,
it currently splices A-->QH before QH-->B ... so that if the HC is
looking at that chunk of schedule at that time, everything starting
at B will be ignored during the rest of that frame.  (Since the QH
is initted to have UHCI_PTR_TERM next, stopping the schedule scan.)

I said "problem" not "bug" since in the current code it would probably
(what does that "PIIX bug" do??) just reduce control/bulk throughput.
That's because the logic is only appending towards the  end of each
frame's schedule, where the FSBR loopback kicks in.