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.c b/drivers/net/sandbox.c
index d538d37..79d0ae6 100644
--- a/drivers/net/sandbox.c
+++ b/drivers/net/sandbox.c
@@ -60,8 +60,9 @@
 
 	debug("eth_sandbox: Start\n");
 
-	fdtdec_get_byte_array(gd->fdt_blob, dev->of_offset, "fake-host-hwaddr",
-			      priv->fake_host_hwaddr, ARP_HLEN);
+	fdtdec_get_byte_array(gd->fdt_blob, dev_of_offset(dev),
+			      "fake-host-hwaddr", priv->fake_host_hwaddr,
+			      ARP_HLEN);
 	priv->recv_packet_buffer = net_rx_packets[0];
 	return 0;
 }