mbox_controller_unregister
mbox_controller_unregister(&mhu->mbox);
EXPORT_SYMBOL_GPL(mbox_controller_unregister);
mbox_controller_unregister(*mbox);
mbox_controller_unregister(&ipcc->mbox);
mbox_controller_unregister(&mbox->controller);
void mbox_controller_unregister(struct mbox_controller *mbox); /* can sleep */