powerpc/fsl: 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: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: linuxppc-dev@lists.ozlabs.org
Link: https://lore.kernel.org/r/20230313182918.1312597-15-gregkh@linuxfoundation.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/powerpc/sysdev/fsl_mpic_timer_wakeup.c

@@ -116,6 +116,7 @@ static struct device_attribute mpic_attributes = __ATTR(timer_wakeup, 0644,
 
 static int __init fsl_wakeup_sys_init(void)
 {
+	struct device *dev_root;
 	int ret;
 
 	fsl_wakeup = kzalloc(sizeof(struct fsl_mpic_timer_wakeup), GFP_KERNEL);
@@ -124,16 +125,26 @@ static int __init fsl_wakeup_sys_init(void)
 
 	INIT_WORK(&fsl_wakeup->free_work, fsl_free_resource);
 
-	ret = device_create_file(mpic_subsys.dev_root, &mpic_attributes);
-	if (ret)
-		kfree(fsl_wakeup);
+	dev_root = bus_get_dev_root(&mpic_subsys);
+	if (dev_root) {
+		ret = device_create_file(dev_root, &mpic_attributes);
+		put_device(dev_root);
+		if (ret)
+			kfree(fsl_wakeup);
+	}
 
 	return ret;
 }
 
 static void __exit fsl_wakeup_sys_exit(void)
 {
-	device_remove_file(mpic_subsys.dev_root, &mpic_attributes);
+	struct device *dev_root;
+
+	dev_root = bus_get_dev_root(&mpic_subsys);
+	if (dev_root) {
+		device_remove_file(dev_root, &mpic_attributes);
+		put_device(dev_root);
+	}
 
 	mutex_lock(&sysfs_lock);