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/mtd/spi/sandbox.c b/drivers/mtd/spi/sandbox.c
index 4944059..36a50fe 100644
--- a/drivers/mtd/spi/sandbox.c
+++ b/drivers/mtd/spi/sandbox.c
@@ -516,7 +516,7 @@
 {
 	struct sandbox_spi_flash_plat_data *pdata = dev_get_platdata(dev);
 	const void *blob = gd->fdt_blob;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 
 	pdata->filename = fdt_getprop(blob, node, "sandbox,filename", NULL);
 	pdata->device_name = fdt_getprop(blob, node, "compatible", NULL);
@@ -641,7 +641,7 @@
 		debug("%s: busnum=%u, cs=%u: binding SPI flash emulation: ",
 		      __func__, busnum, cs);
 		ret = sandbox_sf_bind_emul(state, busnum, cs, bus,
-					   slave->of_offset, slave->name);
+					   dev_of_offset(slave), slave->name);
 		if (ret) {
 			debug("failed (err=%d)\n", ret);
 			return ret;