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/cpsw.c b/drivers/net/cpsw.c
index cbce683..5fbab9e 100644
--- a/drivers/net/cpsw.c
+++ b/drivers/net/cpsw.c
@@ -981,7 +981,7 @@
 
 #ifdef CONFIG_DM_ETH
 	if (slave->data->phy_of_handle)
-		phydev->dev->of_offset = slave->data->phy_of_handle;
+		dev_set_of_offset(phydev->dev, slave->data->phy_of_handle);
 #endif
 
 	priv->phydev = phydev;
@@ -1286,7 +1286,7 @@
 	const char *phy_mode;
 	const char *phy_sel_compat = NULL;
 	const void *fdt = gd->fdt_blob;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 	int subnode;
 	int slave_index = 0;
 	int active_slave;