libata-sff: ap->[last_]ctl are SFF specific

ap->[last_]ctl are specific to SFF controllers.  Put them inside
CONFIG_ATA_SFF and move initialization into ata_sff_port_init().

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