PCI: mediatek-gen3: Fix refcount leak in mtk_pcie_init_irq_domains()

PCI: mediatek-gen3: Fix refcount leak in mtk_pcie_init_irq_domains()

of_get_child_by_name() returns a node pointer with refcount incremented, so
we should use of_node_put() on it when we don't need it anymore.

Add missing of_node_put() to avoid refcount leak.

Fixes: 814cceebba9b ("PCI: mediatek-gen3: Add INTx support")
Link: https://lore.kernel.org/r/20220601041259.56185-1-linmq006@gmail.com
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Miles Chen <miles.chen@mediatek.com>
Acked-by: Jianjun Wang <jianjun.wang@mediatek.com>