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/test/dm/test-fdt.c b/test/dm/test-fdt.c
index 49a36cb..3048a7b 100644
--- a/test/dm/test-fdt.c
+++ b/test/dm/test-fdt.c
@@ -37,9 +37,9 @@
 {
 	struct dm_test_pdata *pdata = dev_get_platdata(dev);
 
-	pdata->ping_add = fdtdec_get_int(gd->fdt_blob, dev->of_offset,
+	pdata->ping_add = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev),
 					"ping-add", -1);
-	pdata->base = fdtdec_get_addr(gd->fdt_blob, dev->of_offset,
+	pdata->base = fdtdec_get_addr(gd->fdt_blob, dev_of_offset(dev),
 				      "ping-expect");
 
 	return 0;
@@ -121,10 +121,10 @@
 		 * want to test the code that sets that up
 		 * (testfdt_drv_probe()).
 		 */
-		base = fdtdec_get_addr(gd->fdt_blob, dev->of_offset,
+		base = fdtdec_get_addr(gd->fdt_blob, dev_of_offset(dev),
 				       "ping-expect");
 		debug("dev=%d, base=%d: %s\n", i, base,
-		      fdt_get_name(gd->fdt_blob, dev->of_offset, NULL));
+		      fdt_get_name(gd->fdt_blob, dev_of_offset(dev), NULL));
 
 		ut_assert(!dm_check_operations(uts, dev, base,
 					       dev_get_priv(dev)));