dm: core: Replace of_offset with accessor (part 2)

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/net/phy/ti.c b/drivers/net/phy/ti.c
index 1d87080..6ad31a0 100644
--- a/drivers/net/phy/ti.c
+++ b/drivers/net/phy/ti.c
@@ -174,7 +174,7 @@
 {
 	struct dp83867_private *dp83867 = phydev->priv;
 	struct udevice *dev = phydev->dev;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 	const void *fdt = gd->fdt_blob;
 
 	if (fdtdec_get_bool(fdt, node, "ti,max-output-impedance"))