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/phy/micrel.c b/drivers/net/phy/micrel.c
index 7163fa2..0e4a4eb 100644
--- a/drivers/net/phy/micrel.c
+++ b/drivers/net/phy/micrel.c
@@ -244,7 +244,7 @@
 		return -EOPNOTSUPP;
 
 	for (i = 0; i < ofcfg->grpsz; i++) {
-		val[i] = fdtdec_get_uint(gd->fdt_blob, dev->of_offset,
+		val[i] = fdtdec_get_uint(gd->fdt_blob, dev_of_offset(dev),
 					 ofcfg->grp[i].name, -1);
 		offset = ofcfg->grp[i].off;
 		if (val[i] == -1) {