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/mmc/exynos_dw_mmc.c b/drivers/mmc/exynos_dw_mmc.c
index c440399..40f7892 100644
--- a/drivers/mmc/exynos_dw_mmc.c
+++ b/drivers/mmc/exynos_dw_mmc.c
@@ -264,7 +264,7 @@
 	struct dwmci_host *host = &priv->host;
 	int err;
 
-	err = exynos_dwmci_get_config(gd->fdt_blob, dev->of_offset, host);
+	err = exynos_dwmci_get_config(gd->fdt_blob, dev_of_offset(dev), host);
 	if (err)
 		return err;
 	err = do_dwmci_init(host);