libata: const-ification bombing run

Enforce access rules where appropriate.

If the compiler is smart enough, this may buy us an optimization or two
as a side effect.
diff --git a/drivers/scsi/sata_sis.c b/drivers/scsi/sata_sis.c
index 0761a32..057f7b9 100644
--- a/drivers/scsi/sata_sis.c
+++ b/drivers/scsi/sata_sis.c
@@ -102,7 +102,7 @@
 	.ordered_flush		= 1,
 };
 
-static struct ata_port_operations sis_ops = {
+static const struct ata_port_operations sis_ops = {
 	.port_disable		= ata_port_disable,
 	.tf_load		= ata_tf_load,
 	.tf_read		= ata_tf_read,