ata_piix: drop merged SCR access and use slave_link instead

Now that libata has slave_link, there's no need to keep ugly merged
SCR access.  Drop it and use slave_link instead.  This results in
simpler code and much better separate link handling for master and
slave.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>