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/xhci-exynos5.c b/drivers/usb/host/xhci-exynos5.c
index 82fcd84..b3f48b3 100644
--- a/drivers/usb/host/xhci-exynos5.c
+++ b/drivers/usb/host/xhci-exynos5.c
@@ -68,7 +68,7 @@
 	}
 
 	depth = 0;
-	node = fdtdec_next_compatible_subnode(blob, dev->of_offset,
+	node = fdtdec_next_compatible_subnode(blob, dev_of_offset(dev),
 				COMPAT_SAMSUNG_EXYNOS5_USB3_PHY, &depth);
 	if (node <= 0) {
 		debug("XHCI: Can't get device node for usb3-phy controller\n");