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/xilinx_emaclite.c b/drivers/net/xilinx_emaclite.c
index ea93cf9..7d4e14f 100644
--- a/drivers/net/xilinx_emaclite.c
+++ b/drivers/net/xilinx_emaclite.c
@@ -604,15 +604,15 @@
 
 	emaclite->phyaddr = -1;
 
-	offset = fdtdec_lookup_phandle(gd->fdt_blob, dev->of_offset,
+	offset = fdtdec_lookup_phandle(gd->fdt_blob, dev_of_offset(dev),
 				      "phy-handle");
 	if (offset > 0)
 		emaclite->phyaddr = fdtdec_get_int(gd->fdt_blob, offset,
 						   "reg", -1);
 
-	emaclite->txpp = fdtdec_get_int(gd->fdt_blob, dev->of_offset,
+	emaclite->txpp = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev),
 					"xlnx,tx-ping-pong", 0);
-	emaclite->rxpp = fdtdec_get_int(gd->fdt_blob, dev->of_offset,
+	emaclite->rxpp = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev),
 					"xlnx,rx-ping-pong", 0);
 
 	printf("EMACLITE: %lx, phyaddr %d, %d/%d\n", (ulong)emaclite->regs,