mailbox: constify mbox_chan_ops structure

This mbox_chan_ops structure is never modified, so declare it as const,
like all the other mbox_chan_ops structures.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <[e-mail]>
Signed-off-by: Jassi Brar <[e-mail]>

drivers/mailbox/mailbox-sti.c

···
 	return chan;
 }
 
-static struct mbox_chan_ops sti_mbox_ops = {
+static const struct mbox_chan_ops sti_mbox_ops = {
 	.startup	= sti_mbox_startup_chan,
 	.shutdown	= sti_mbox_shutdown_chan,
 	.send_data	= sti_mbox_send_data,