mailbox: zynq: add missing of_node_put before return

Fix following coccicheck warning:
WARNING: Function "for_each_available_child_of_node"
should have of_node_put() before return.

Early exits from for_each_available_child_of_node should decrement the
node reference counter.

Signed-off-by: Wang Qing <[e-mail]>
Signed-off-by: Jassi Brar <[e-mail]>

drivers/mailbox/zynqmp-ipi-mailbox.c

···
 		mbox->pdata = pdata;
 		ret = zynqmp_ipi_mbox_probe(mbox, nc);
 		if (ret) {
+			of_node_put(nc);
 			dev_err(dev, "failed to probe subdev.\n");
 			ret = -EINVAL;
 			goto free_mbox_dev;