libata: fix build without BMDMA

fix these errors:
drivers/ata/libata-sff.c:2538:3: error: implicit declaration of function
'ata_pci_bmdma_prepare_host'
drivers/ata/libata-sff.c:2549:40: error: 'ata_bmdma_interrupt'
undeclared (first use in this function)

Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>