use PIO (or equivalent) code to do asynchronous transfers: In
ncr5380_scsi_cmd(), test to see if the request has been completed after
the call to ncr5380_sched(), and return COMPLETE if so. This avoids
going into an infinite loop in scsipi_execute_xs() while waiting for an
interrupt to trigger completion of the transfer... which, of course,
never happens, since it's already done.