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/sandbox-raw.c b/drivers/net/sandbox-raw.c
index 5912427..9742c2c 100644
--- a/drivers/net/sandbox-raw.c
+++ b/drivers/net/sandbox-raw.c
@@ -26,7 +26,7 @@
 
 	debug("eth_sandbox_raw: Start\n");
 
-	interface = fdt_getprop(gd->fdt_blob, dev->of_offset,
+	interface = fdt_getprop(gd->fdt_blob, dev_of_offset(dev),
 					    "host-raw-interface", NULL);
 	if (interface == NULL)
 		return -EINVAL;