mailbox: imx: Fix return in imx_mu_scu_xlate()

This called from mbox_request_channel().  The caller is  expecting error
pointers and not NULL so this "return NULL;" will lead to an Oops.

Fixes: 0a67003b ("mailbox: imx: add SCU MU support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/imx-mailbox.c

@@ -378,7 +378,7 @@ static struct mbox_chan *imx_mu_scu_xlate(struct mbox_controller *mbox,
 		break;
 	default:
 		dev_err(mbox->dev, "Invalid chan type: %d\n", type);
-		return NULL;
+		return ERR_PTR(-EINVAL);
 	}
 
 	if (chan >= mbox->num_chans) {