libata: implement and use ata_noop_irq_clear()

->irq_clear() is used to clear IRQ bit of a SFF controller and isn't
useful for drivers which don't use libata SFF HSM implementation.
However, it's a required callback and many drivers implement their own
noop version as placeholder.  This patch implements ata_noop_irq_clear
and use it to replace those custom placeholders.

Also, SFF drivers which don't support BMDMA don't need to use
ata_bmdma_irq_clear().  It becomes noop if BMDMA address isn't
initialized.  Convert them to use ata_noop_irq_clear().

Signed-off-by: Tejun Heo <htejun@gmail.com>

drivers/ata/libata-sff.c

@@ -302,6 +302,14 @@ void ata_bmdma_irq_clear(struct ata_port *ap)
 	iowrite8(ioread8(mmio + ATA_DMA_STATUS), mmio + ATA_DMA_STATUS);
 }
 
+/**
+ *	ata_noop_irq_clear - Noop placeholder for irq_clear
+ *	@ap: Port associated with this ATA transaction.
+ */
+void ata_noop_irq_clear(struct ata_port *ap)
+{
+}
+
 /**
  *	ata_bmdma_status - Read PCI IDE BMDMA status
  *	@ap: Port associated with this ATA transaction.