ahci: Check cmd_fis[1] more explicitly

Instead of checking for a known byte, inspect the
fields of this byte explicitly to produce more meaningful
error messages and improve the readability of this section.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1415058979-16604-5-git-send-email-jsnow@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
