This patch makes EHCI

  - Share the same TD queueing code for control, bulk,
    and interrupt traffic;
  - Queue interrupt transfers, modifying the code for
    urb submit/unlink/complete;
  - Thinner, by removing lots of nasty fatty special case
    logic for interrupt transfers (size, no queueing, etc);
  - Grow some "automagic resubmit" logic, ready to be
    ripped out soonish;
  - Package its interrupt scheduling so it can be called
    from more places.