libata: reorder ata_queued_cmd to remove alignment padding on 64 bit builds

Reorder structure ata_queued_cmd to remove 8 bytes of alignment padding
on 64 bit builds & therefore reduce the size of structure ata_port by
256 bytes.

Overall this will have little impact, other than reducing the amount of
memory that is cleared when allocating ata_ports.

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>