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/pata_platform.c b/drivers/ata/pata_platform.c
index 1edfc13..6527c56 100644
--- a/drivers/ata/pata_platform.c
+++ b/drivers/ata/pata_platform.c
@@ -52,7 +52,7 @@
 
 static struct ata_port_operations pata_platform_port_ops = {
 	.inherits		= &ata_sff_port_ops,
-	.data_xfer		= ata_sff_data_xfer_noirq,
+	.sff_data_xfer		= ata_sff_data_xfer_noirq,
 	.cable_detect		= ata_cable_unknown,
 	.set_mode		= pata_platform_set_mode,
 	.port_start		= ATA_OP_NULL,