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>

drivers/ata/pata_cmd640.c

@@ -172,7 +172,7 @@ static struct scsi_host_template cmd640_sht = {
 static struct ata_port_operations cmd640_port_ops = {
 	.inherits	= &ata_bmdma_port_ops,
 	/* In theory xfer_noirq is not needed once we kill the prefetcher */
-	.data_xfer	= ata_sff_data_xfer_noirq,
+	.sff_data_xfer	= ata_sff_data_xfer_noirq,
 	.qc_issue	= cmd640_qc_issue,
 	.cable_detect	= ata_cable_40wire,
 	.set_piomode	= cmd640_set_piomode,