mailbox: Make mbox_chan_ops const

The mailbox controller's channel ops ought to be read-only.  Update
all the mailbox drivers to make their mbox_chan_ops const as well.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Cc: Ashwin Chaugule <ashwin.chaugule@linaro.org>
Cc: Ley Foon Tan <lftan@altera.com>
Acked-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
diff --git a/drivers/mailbox/arm_mhu.c b/drivers/mailbox/arm_mhu.c
index ac693c6..d9e99f9 100644
--- a/drivers/mailbox/arm_mhu.c
+++ b/drivers/mailbox/arm_mhu.c
@@ -110,7 +110,7 @@
 	free_irq(mlink->irq, chan);
 }
 
-static struct mbox_chan_ops mhu_ops = {
+static const struct mbox_chan_ops mhu_ops = {
 	.send_data = mhu_send_data,
 	.startup = mhu_startup,
 	.shutdown = mhu_shutdown,