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/spi/ich.c b/drivers/spi/ich.c
index caf0103..893fe33 100644
--- a/drivers/spi/ich.c
+++ b/drivers/spi/ich.c
@@ -658,14 +658,14 @@
 static int ich_spi_ofdata_to_platdata(struct udevice *dev)
 {
 	struct ich_spi_platdata *plat = dev_get_platdata(dev);
+	int node = dev_of_offset(dev);
 	int ret;
 
-	ret = fdt_node_check_compatible(gd->fdt_blob, dev->of_offset,
-					"intel,ich7-spi");
+	ret = fdt_node_check_compatible(gd->fdt_blob, node, "intel,ich7-spi");
 	if (ret == 0) {
 		plat->ich_version = ICHV_7;
 	} else {
-		ret = fdt_node_check_compatible(gd->fdt_blob, dev->of_offset,
+		ret = fdt_node_check_compatible(gd->fdt_blob, node,
 						"intel,ich9-spi");
 		if (ret == 0)
 			plat->ich_version = ICHV_9;