- Delays or eliminates some IRQs  It'll mostly affect control
   or iso transfers, which typically have multiple TDs per URB,
   by making only the last TD generate an IRQ.

- Shortens some of the submit path that needs to run with
   IRQs disabled ... no need to use the dma_to_td hashtable.
   (Of course that path is still pretty long...)

- Gets rid of case where the ED state was confused ... now
   there's only one such state, not two.