q800: don't access Nubus bus directly from the mac-nubus-bridge device

Instead use the qdev_get_child_bus() function which is intended for this exact
purpose.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230621085353.113233-22-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

hw/m68k/q800.c

@@ -448,7 +448,7 @@ static void q800_machine_init(MachineState *machine)
                           qdev_get_gpio_in_named(DEVICE(&m->via2), "nubus-irq",
                                                  VIA2_NUBUS_IRQ_9));
 
-    nubus = &NUBUS_BRIDGE(dev)->bus;
+    nubus = NUBUS_BUS(qdev_get_child_bus(dev, "nubus-bus.0"));
 
     /* framebuffer in nubus slot #9 */