libata: factor out ata_pci_activate_sff_host() from ata_pci_one()

Factor out ata_pci_activate_sff_host() from ata_pci_one().  This does
about the same thing as ata_host_activate() but needs to be separate
because SFF controllers use different and multiple IRQs in legacy
mode.

This will be used to make SFF LLD initialization more flexible.

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