o Switch to handling bad SCSI status as a sequencer interrupt
   instead of having the kernel proccess these failures via
   the completion queue.  This is done because:

    - The old scheme required us to pause the sequencer and clear
      critical sections for each SCB.  It seems that these pause
      actions, if coincident with a sequencer FIFO interrupt, would
      result in a FIFO interrupt getting lost or directing to the
      wrong FIFO.  This caused hangs when the driver was stressed
      under high "queue full" loads.
    - The completion code assumed that it was always called with
      the sequencer running.  This may not be the case in timeout
      processing where completions occur manually via
      ahd_pause_and_flushwork().
    - With this scheme, the extra expense of clearing critical
      sections is avoided since the sequencer will only self pause
      once all pending selections have cleared and it is not in
      a critical section.