ata_piix: SITRE handling fix

Set SITRE (separate slave timing register) bit also in master-only
configurations to match other PIIX-alike drivers and make further
changes easier.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>