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/cadence_qspi.c b/drivers/spi/cadence_qspi.c
index f16f90d..9a6e41f 100644
--- a/drivers/spi/cadence_qspi.c
+++ b/drivers/spi/cadence_qspi.c
@@ -282,7 +282,7 @@
 {
 	struct cadence_spi_platdata *plat = bus->platdata;
 	const void *blob = gd->fdt_blob;
-	int node = bus->of_offset;
+	int node = dev_of_offset(bus);
 	int subnode;
 	u32 data[4];
 	int ret;