libata-core/sff: Fix multiple assumptions about DMA

The ata IRQ ack functions are only used when debugging. Unfortunately
almost every controller that calls them can cause crashes in some
configurations as there are missing checks for bmdma presence.

In addition ata_port_start insists of installing DMA buffers and pad
buffers for controllers regardless. The SFF controllers actually need to
make that decision dynamically at controller setup time and all need the
same helper - so we add ata_sff_port_start. Future patches will switch
the SFF drivers to use this.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>