ahci: add ncq debug checks

Most of the time, these bits can be safely ignored. For the purposes
of debugging however, it's nice to know that they're not being used.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1435016308-6150-7-git-send-email-jsnow@redhat.com
