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/spi/designware_spi.c b/drivers/spi/designware_spi.c
index 24a6e98..9d5e29c 100644
--- a/drivers/spi/designware_spi.c
+++ b/drivers/spi/designware_spi.c
@@ -132,7 +132,7 @@
 {
 	struct dw_spi_platdata *plat = bus->platdata;
 	const void *blob = gd->fdt_blob;
-	int node = bus->of_offset;
+	int node = dev_of_offset(bus);
 
 	plat->regs = (struct dw_spi *)dev_get_addr(bus);