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

In the Linux kernel, the following vulnerability has been resolved: 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.