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/i2c/cros_ec_tunnel.c b/drivers/i2c/cros_ec_tunnel.c
index 9cf8e7d..86fa684 100644
--- a/drivers/i2c/cros_ec_tunnel.c
+++ b/drivers/i2c/cros_ec_tunnel.c
@@ -34,7 +34,7 @@
 {
 	struct cros_ec_i2c_bus *i2c_bus = dev_get_priv(dev);
 	const void *blob = gd->fdt_blob;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 
 	i2c_bus->remote_bus = fdtdec_get_uint(blob, node, "google,remote-bus",
 					      0);