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/ag7xxx.c b/drivers/net/ag7xxx.c
index 346f138..f8782bc 100644
--- a/drivers/net/ag7xxx.c
+++ b/drivers/net/ag7xxx.c
@@ -857,7 +857,7 @@
 {
 	int offset;
 
-	offset = fdtdec_lookup_phandle(gd->fdt_blob, dev->of_offset, "phy");
+	offset = fdtdec_lookup_phandle(gd->fdt_blob, dev_of_offset(dev), "phy");
 	if (offset <= 0) {
 		debug("%s: PHY OF node not found (ret=%i)\n", __func__, offset);
 		return -EINVAL;