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/usb/host/ehci-vf.c b/drivers/usb/host/ehci-vf.c
index f389bff..84241cd 100644
--- a/drivers/usb/host/ehci-vf.c
+++ b/drivers/usb/host/ehci-vf.c
@@ -218,7 +218,7 @@
 {
 	struct ehci_vf_priv_data *priv = dev_get_priv(dev);
 	const void *dt_blob = gd->fdt_blob;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 	const char *mode;
 
 	priv->portnr = dev->seq;