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>
diff --git a/drivers/ata/pata_at32.c b/drivers/ata/pata_at32.c
index db057b1..27c959f 100644
--- a/drivers/ata/pata_at32.c
+++ b/drivers/ata/pata_at32.c
@@ -166,11 +166,6 @@
 	}
 }
 
-static void pata_at32_irq_clear(struct ata_port *ap)
-{
-	/* No DMA controller yet */
-}
-
 static struct scsi_host_template at32_sht = {
 	.module			= THIS_MODULE,
 	.name			= DRV_NAME,
@@ -208,7 +203,7 @@
 
 	.data_xfer		= ata_data_xfer,
 
-	.irq_clear		= pata_at32_irq_clear,
+	.irq_clear		= ata_noop_irq_clear,
 	.irq_on			= ata_irq_on,
 
 	.port_start		= ata_sff_port_start,