ahci: Do not map cmd_fis to generate response

ahci: Do not map cmd_fis to generate response

The Register D2H FIS should copy the current values of
the registers instead of just parroting back the same
values the guest sent back to it.

In this case, the SECTOR COUNT variables are actually
not generally meaningful in terms of standard commands
(See ATA8-AC3 Section 9.2 Normal Outputs), so it actually
probably doesn't matter what we put in here.

Meanwhile, we do need to use the Register update FIS from
the NCQ pathways (in error cases), so getting rid of
references to cur_cmd here is a win for AHCI concurrency.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1435767578-32743-14-git-send-email-jsnow@redhat.com