libata-sff: ata_sff_irq_clear() is BMDMA specific

ata_sff_irq_clear() is BMDMA specific.  Rename it to
ata_bmdma_irq_clear(), move it to ata_bmdma_port_ops and make
->sff_irq_clear() optional.

Note: ata_bmdma_irq_clear() is actually only needed by ata_piix and
      possibly by sata_sil.  This should be moved to respective low
      level drivers later.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
diff --git a/drivers/ata/pata_bf54x.c b/drivers/ata/pata_bf54x.c
index 6422cfd..9cae65d 100644
--- a/drivers/ata/pata_bf54x.c
+++ b/drivers/ata/pata_bf54x.c
@@ -1214,7 +1214,7 @@
  *	bfin_irq_clear - Clear ATAPI interrupt.
  *	@ap: Port associated with this ATA transaction.
  *
- *	Note: Original code is ata_sff_irq_clear().
+ *	Note: Original code is ata_bmdma_irq_clear().
  */
 
 static void bfin_irq_clear(struct ata_port *ap)