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/i2c/muxes/i2c-mux-uclass.c b/drivers/i2c/muxes/i2c-mux-uclass.c
index db086ef..d243b8e 100644
--- a/drivers/i2c/muxes/i2c-mux-uclass.c
+++ b/drivers/i2c/muxes/i2c-mux-uclass.c
@@ -40,7 +40,7 @@
 	struct i2c_mux_bus *plat = dev_get_parent_platdata(dev);
 	int channel;
 
-	channel = fdtdec_get_int(gd->fdt_blob, dev->of_offset, "reg", -1);
+	channel = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev), "reg", -1);
 	if (channel < 0)
 		return -EINVAL;
 	plat->channel = channel;
@@ -60,7 +60,7 @@
 	 * There is no compatible string in the sub-nodes, so we must manually
 	 * bind these
 	 */
-	for (offset = fdt_first_subnode(blob, mux->of_offset);
+	for (offset = fdt_first_subnode(blob, dev_of_offset(mux));
 	     offset > 0;
 	     offset = fdt_next_subnode(blob, offset)) {
 		struct udevice *dev;