It fixes problems with interrupt transfers, which I think that
nobody else has run into (or I'd surely have heard of it :).
Looks like not many folk are using USB 2.0 hubs yet.

    - wasn't checking enough of the periodic schedule to
      detect bandwidth overcommit (would BUG out)
    - frames to uframes is rightshift 3, not 8 :)
    - properly cleans up (no oops!) after certain rare errors
      in the interrupt submit path (just my luck to hit one)
    - use that cleanup to bypass some old implementation
      shortcuts in the control and bulk submit paths
    - there are also some other minor updates/cleanups