libata-sff: introduce ata_sff_init/exit() and ata_sff_port_init()

In preparation of proper SFF/BMDMA separation, introduce
ata_sff_init/exit() and ata_sff_port_init().  These functions
currently don't do anything.

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