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>

drivers/usb/emul/sandbox_hub.c

@@ -277,7 +277,8 @@ static int sandbox_child_post_bind(struct udevice *dev)
 {
 	struct sandbox_hub_platdata *plat = dev_get_parent_platdata(dev);
 
-	plat->port = fdtdec_get_int(gd->fdt_blob, dev->of_offset, "reg", -1);
+	plat->port = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev), "reg",
+				    -1);
 
 	return 0;
 }