libata/sff: Use ops->bmdma_stop instead of ata_bmdma_stop()

In libata-sff, ata_sff_post_internal_cmd() directly calls ata_bmdma_stop()
instead of ap->ops->bmdma_stop(). This can be a problem for controllers
that use their own bmdma_stop for which the generic sff one isn't suitable

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>