irqchip/loongson-pch-msi: Use irq_domain_create_hierarchy()

Use the irq_domain_create_hierarchy() helper to create the hierarchical
domain, which both serves as documentation and avoids poking at
irqdomain internals.

Reviewed-by: Philippe Mathieu-Daudé <[e-mail]>
Tested-by: Hsin-Yi Wang <[e-mail]>
Tested-by: Mark-PK Tsai <[e-mail]>
Signed-off-by: Johan Hovold <[e-mail]>
Signed-off-by: Marc Zyngier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/irqchip/irq-loongson-pch-msi.c

···
 {
 	struct irq_domain *middle_domain, *msi_domain;
 
-	middle_domain = irq_domain_create_linear(domain_handle,
-						priv->num_irqs,
-						&pch_msi_middle_domain_ops,
-						priv);
+	middle_domain = irq_domain_create_hierarchy(parent, 0, priv->num_irqs,
+						    domain_handle,
+						    &pch_msi_middle_domain_ops,
+						    priv);
 	if (!middle_domain) {
 		pr_err("Failed to create the MSI middle domain\n");
 		return -ENOMEM;
 	}
 
-	middle_domain->parent = parent;
 	irq_domain_update_bus_token(middle_domain, DOMAIN_BUS_NEXUS);
 
 	msi_domain = pci_msi_create_irq_domain(domain_handle,