Description:
  After calling the completion callback, the libata error handler might be
running and getting atapi sense data. Clearing the ATA_QCFLAG_ACTIVE flag
at this point might interfere with the libata error handler.

Changes:
   - Clear the ATA_QCFLAG_ACTIVE flag before calling the completion callback
     (and also before the error handler)
   - Add some comment
Signed-off-by: Albert Lee <albertcc@tw.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>