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 <Julia.Lawall@lip6.fr>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>