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/misc/i2c_eeprom_emul.c b/drivers/misc/i2c_eeprom_emul.c
index 4410d03..02de8d7 100644
--- a/drivers/misc/i2c_eeprom_emul.c
+++ b/drivers/misc/i2c_eeprom_emul.c
@@ -115,9 +115,9 @@
 {
 	struct sandbox_i2c_flash_plat_data *plat = dev_get_platdata(dev);
 
-	plat->size = fdtdec_get_int(gd->fdt_blob, dev->of_offset,
+	plat->size = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev),
 				    "sandbox,size", 32);
-	plat->filename = fdt_getprop(gd->fdt_blob, dev->of_offset,
+	plat->filename = fdt_getprop(gd->fdt_blob, dev_of_offset(dev),
 				     "sandbox,filename", NULL);
 	if (!plat->filename) {
 		debug("%s: No filename for device '%s'\n", __func__,