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_sil24.c b/drivers/scsi/sata_sil24.c
index 1985781..32d730b 100644
--- a/drivers/scsi/sata_sil24.c
+++ b/drivers/scsi/sata_sil24.c
@@ -275,7 +275,7 @@
 	.ordered_flush		= 1, /* NCQ not supported yet */
 };
 
-static struct ata_port_operations sil24_ops = {
+static const struct ata_port_operations sil24_ops = {
 	.port_disable		= ata_port_disable,
 
 	.check_status		= sil24_check_status,