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/sata_qstor.c b/drivers/ata/sata_qstor.c
index 91cc12c..3c8e97f 100644
--- a/drivers/ata/sata_qstor.c
+++ b/drivers/ata/sata_qstor.c
@@ -121,7 +121,6 @@
 static int qs_check_atapi_dma(struct ata_queued_cmd *qc);
 static void qs_bmdma_stop(struct ata_queued_cmd *qc);
 static u8 qs_bmdma_status(struct ata_port *ap);
-static void qs_irq_clear(struct ata_port *ap);
 static void qs_freeze(struct ata_port *ap);
 static void qs_thaw(struct ata_port *ap);
 static void qs_error_handler(struct ata_port *ap);
@@ -157,7 +156,7 @@
 	.freeze			= qs_freeze,
 	.thaw			= qs_thaw,
 	.error_handler		= qs_error_handler,
-	.irq_clear		= qs_irq_clear,
+	.irq_clear		= ata_noop_irq_clear,
 	.irq_on			= ata_irq_on,
 	.scr_read		= qs_scr_read,
 	.scr_write		= qs_scr_write,
@@ -211,11 +210,6 @@
 	return 0;
 }
 
-static void qs_irq_clear(struct ata_port *ap)
-{
-	/* nothing */
-}
-
 static inline void qs_enter_reg_mode(struct ata_port *ap)
 {
 	u8 __iomem *chan = qs_mmio_base(ap->host) + (ap->port_no * 0x4000);