ata_piix: kill ich6_sata_ahci and clean up

ich6_sata_ahci and ich6_sata are identical.  Kill ich6_sata_ahci and
drop _ahci postfixes from controller ids, which doesn't really mean
anything at this point.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>