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/usb-uclass.c b/drivers/usb/host/usb-uclass.c
index be114fc..5cf1e9a 100644
--- a/drivers/usb/host/usb-uclass.c
+++ b/drivers/usb/host/usb-uclass.c
@@ -686,16 +686,17 @@
 	const void *blob = gd->fdt_blob;
 	int val;
 
-	if (dev->of_offset == -1)
+	if (dev_of_offset(dev) == -1)
 		return 0;
 
 	/* We only support matching a few things */
-	val = fdtdec_get_int(blob, dev->of_offset, "usb,device-class", -1);
+	val = fdtdec_get_int(blob, dev_of_offset(dev), "usb,device-class", -1);
 	if (val != -1) {
 		plat->id.match_flags |= USB_DEVICE_ID_MATCH_DEV_CLASS;
 		plat->id.bDeviceClass = val;
 	}
-	val = fdtdec_get_int(blob, dev->of_offset, "usb,interface-class", -1);
+	val = fdtdec_get_int(blob, dev_of_offset(dev), "usb,interface-class",
+			     -1);
 	if (val != -1) {
 		plat->id.match_flags |= USB_DEVICE_ID_MATCH_INT_CLASS;
 		plat->id.bInterfaceClass = val;