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_qdi.c b/drivers/ata/pata_qdi.c
index 0d81dd5..bf45cf0 100644
--- a/drivers/ata/pata_qdi.c
+++ b/drivers/ata/pata_qdi.c
@@ -160,7 +160,7 @@
 static struct ata_port_operations qdi6500_port_ops = {
 	.inherits	= &ata_sff_port_ops,
 	.qc_issue	= qdi_qc_issue,
-	.data_xfer	= qdi_data_xfer,
+	.sff_data_xfer	= qdi_data_xfer,
 	.cable_detect	= ata_cable_40wire,
 	.set_piomode	= qdi6500_set_piomode,
 };