ahci: check for ncq prdtl overflow

Don't attempt the NCQ transfer if the PRDT we were given is not big
enough to perform the entire transfer.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1435016308-6150-5-git-send-email-jsnow@redhat.com
