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/net/mvneta.c b/drivers/net/mvneta.c
index 00f378f..674075f 100644
--- a/drivers/net/mvneta.c
+++ b/drivers/net/mvneta.c
@@ -1611,7 +1611,7 @@
 	struct eth_pdata *pdata = dev_get_platdata(dev);
 	struct mvneta_port *pp = dev_get_priv(dev);
 	void *blob = (void *)gd->fdt_blob;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 	struct mii_dev *bus;
 	unsigned long addr;
 	void *bd_space;
@@ -1691,7 +1691,8 @@
 
 	/* Get phy-mode / phy_interface from DT */
 	pdata->phy_interface = -1;
-	phy_mode = fdt_getprop(gd->fdt_blob, dev->of_offset, "phy-mode", NULL);
+	phy_mode = fdt_getprop(gd->fdt_blob, dev_of_offset(dev), "phy-mode",
+			       NULL);
 	if (phy_mode)
 		pdata->phy_interface = phy_get_interface_by_name(phy_mode);
 	if (pdata->phy_interface == -1) {