atapi: Prioritize unknown cmd error over BCL error

If we don't know about the command at all, we need to prioritize
that failure above the zero byte-count-limit failure.

This fixes a failure in the sparc64 NetBSD 7.0 installer bootup.

Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: John Snow <jsnow@redhat.com>
Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-id: 1447095959-10046-3-git-send-email-jsnow@redhat.com
