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/altera_qspi.c b/drivers/mtd/altera_qspi.c
index a9148a7..e04964b 100644
--- a/drivers/mtd/altera_qspi.c
+++ b/drivers/mtd/altera_qspi.c
@@ -349,7 +349,7 @@
 {
 	struct altera_qspi_platdata *pdata = dev_get_platdata(dev);
 	void *blob = (void *)gd->fdt_blob;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 	const char *list, *end;
 	const fdt32_t *cell;
 	void *base;