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/pata_platform.c

@@ -86,7 +86,7 @@ static struct ata_port_operations pata_platform_port_ops = {
 
 	.data_xfer		= ata_data_xfer_noirq,
 
-	.irq_clear		= ata_bmdma_irq_clear,
+	.irq_clear		= ata_noop_irq_clear,
 	.irq_on			= ata_irq_on,
 
 	.port_start		= ata_dummy_ret0,