- convert the audio softintr's to SOFTINT_SERIAL.  right now they're on
  the same priority level as USB and if an audio softintr is blocked
  waiting for a usb event, they'll dead lock.  also, audio being higher
  priority is a fine idea.

- revert the previous audio.c changes as they were unsafe and this change
  obviates the need for them.

XXX:  convert the audio softint's to use a high priority thread or a
XXX:  workqueue would be better again.

(mrg)