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/gmac_rockchip.c b/drivers/net/gmac_rockchip.c
index 5f833fa..e9b202a 100644
--- a/drivers/net/gmac_rockchip.c
+++ b/drivers/net/gmac_rockchip.c
@@ -36,9 +36,9 @@
 {
 	struct gmac_rockchip_platdata *pdata = dev_get_platdata(dev);
 
-	pdata->tx_delay = fdtdec_get_int(gd->fdt_blob, dev->of_offset,
+	pdata->tx_delay = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev),
 					 "tx-delay", 0x30);
-	pdata->rx_delay = fdtdec_get_int(gd->fdt_blob, dev->of_offset,
+	pdata->rx_delay = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev),
 					 "rx-delay", 0x10);
 
 	return designware_eth_ofdata_to_platdata(dev);