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>