mailbox: hi3660: constify mbox_chan_ops structure

The mbox_chan_ops structure can be const as it is only stored in the
ops field of an mbox_controller structure and this field is const.

Done with the help of Coccinelle.
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/hi3660-mailbox.c

@@ -206,7 +206,7 @@ static int hi3660_mbox_send_data(struct mbox_chan *chan, void *msg)
 	return 0;
 }
 
-static struct mbox_chan_ops hi3660_mbox_ops = {
+static const struct mbox_chan_ops hi3660_mbox_ops = {
 	.startup	= hi3660_mbox_startup,
 	.send_data	= hi3660_mbox_send_data,
 };