irqchip/mbigen: move to use bus_get_dev_root()

Direct access to the struct bus_type dev_root pointer is going away soon
so replace that with a call to bus_get_dev_root() instead, which is what
it is there for.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230313182918.1312597-21-gregkh@linuxfoundation.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/irqchip/irq-mbigen.c

@@ -245,11 +245,14 @@ static int mbigen_of_create_domain(struct platform_device *pdev,
 		if (!of_property_read_bool(np, "interrupt-controller"))
 			continue;
 
-		parent = platform_bus_type.dev_root;
-		child = of_platform_device_create(np, NULL, parent);
-		if (!child) {
-			of_node_put(np);
-			return -ENOMEM;
+		parent = bus_get_dev_root(&platform_bus_type);
+		if (parent) {
+			child = of_platform_device_create(np, NULL, parent);
+			put_device(parent);
+			if (!child) {
+				of_node_put(np);
+				return -ENOMEM;
+			}
 		}
 
 		if (of_property_read_u32(child->dev.of_node, "num-pins",