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/s5p_sdhci.c b/drivers/mmc/s5p_sdhci.c
index 28327d5..640ea02 100644
--- a/drivers/mmc/s5p_sdhci.c
+++ b/drivers/mmc/s5p_sdhci.c
@@ -247,7 +247,7 @@
 	struct sdhci_host *host = dev_get_priv(dev);
 	int ret;
 
-	ret = sdhci_get_config(gd->fdt_blob, dev->of_offset, host);
+	ret = sdhci_get_config(gd->fdt_blob, dev_of_offset(dev), host);
 	if (ret)
 		return ret;