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/sysreset/sysreset_sandbox.c b/drivers/sysreset/sysreset_sandbox.c
index 7ae7f38..0c4e2e1 100644
--- a/drivers/sysreset/sysreset_sandbox.c
+++ b/drivers/sysreset/sysreset_sandbox.c
@@ -41,7 +41,7 @@
 	 * (see the U_BOOT_DEVICE() declaration below) should not do anything.
 	 * If we are that device, return an error.
 	 */
-	if (state->fdt_fname && dev->of_offset == -1)
+	if (state->fdt_fname && dev_of_offset(dev) == -1)
 		return -ENODEV;
 
 	switch (type) {