dm: core: Replace of_offset with accessor

At present devices use a simple integer offset to record the device tree
node associated with the device. In preparation for supporting a live
device tree, which uses a node pointer instead, refactor existing code to
access this field through an inline function.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/core/root.c b/drivers/core/root.c
index 9edfc1e..175fd3f 100644
--- a/drivers/core/root.c
+++ b/drivers/core/root.c
@@ -227,10 +227,10 @@
 
 int dm_scan_fdt_dev(struct udevice *dev)
 {
-	if (dev->of_offset == -1)
+	if (dev_of_offset(dev) == -1)
 		return 0;
 
-	return dm_scan_fdt_node(dev, gd->fdt_blob, dev->of_offset,
+	return dm_scan_fdt_node(dev, gd->fdt_blob, dev_of_offset(dev),
 				gd->flags & GD_FLG_RELOC ? false : true);
 }