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/usb/host/dwc2.c b/drivers/usb/host/dwc2.c
index d08879d..d253b94 100644
--- a/drivers/usb/host/dwc2.c
+++ b/drivers/usb/host/dwc2.c
@@ -1199,8 +1199,8 @@
 		return -EINVAL;
 	priv->regs = (struct dwc2_core_regs *)addr;
 
-	prop = fdt_getprop(gd->fdt_blob, dev->of_offset, "disable-over-current",
-			   NULL);
+	prop = fdt_getprop(gd->fdt_blob, dev_of_offset(dev),
+			   "disable-over-current", NULL);
 	if (prop)
 		priv->oc_disable = true;