mailbox: bcm2835: Fix of_xlate return value

The bcm2835-mailbox returns NULL instead of an error pointer, which could
result in a NULL ptr dereference in mbox_request_channel. So fix this
by returning a proper error pointer.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Fixes: 0bae6af6d704 ("mailbox: Enable BCM2835 mailbox support")
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/bcm2835-mailbox.c

@@ -134,7 +134,7 @@ static struct mbox_chan *bcm2835_mbox_index_xlate(struct mbox_controller *mbox,
 		    const struct of_phandle_args *sp)
 {
 	if (sp->args_count != 0)
-		return NULL;
+		return ERR_PTR(-EINVAL);
 
 	return &mbox->chans[0];
 }