libata: rename SFF port ops

Add sff_ prefix to SFF specific port ops.

This rename is in preparation of separating SFF support out of libata
core layer.  This patch strictly renames ops and doesn't introduce any
behavior difference.

Signed-off-by: Tejun Heo <htejun@gmail.com>
diff --git a/drivers/ata/sata_promise.c b/drivers/ata/sata_promise.c
index 9923e860..5a10dc5 100644
--- a/drivers/ata/sata_promise.c
+++ b/drivers/ata/sata_promise.c
@@ -162,12 +162,12 @@
 static const struct ata_port_operations pdc_common_ops = {
 	.inherits		= &ata_sff_port_ops,
 
-	.tf_load		= pdc_tf_load_mmio,
-	.exec_command		= pdc_exec_command_mmio,
+	.sff_tf_load		= pdc_tf_load_mmio,
+	.sff_exec_command	= pdc_exec_command_mmio,
 	.check_atapi_dma	= pdc_check_atapi_dma,
 	.qc_prep		= pdc_qc_prep,
 	.qc_issue		= pdc_qc_issue,
-	.irq_clear		= pdc_irq_clear,
+	.sff_irq_clear		= pdc_irq_clear,
 
 	.post_internal_cmd	= pdc_post_internal_cmd,
 	.error_handler		= pdc_error_handler,