AHCI - Fix panic if additional I/O is queued during SATA error processing.

* If additional I/O is queued during SATA error processing the AHCI
  driver was improperly trying to initiate the new I/O.  This caused
  the error processing code to assert on unexpected command activity.

* Fix by implying exclusive access mode when the error CCB is in use and
  giving the error CCB queueing priority over all other CCBs.