ahci: assert is_ncq for process_ncq

We already checked this in the handle_cmd phase, so just
change this to an assertion and simplify the error logic.

(Also, fix the switch indent, because checkpatch.pl yelled.)
((Sorry for churn.))

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1435767578-32743-4-git-send-email-jsnow@redhat.com
