dm: core: Replace of_offset with accessor (part 2)

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/ti_qspi.c b/drivers/spi/ti_qspi.c
index 0e8411b..bea3aff 100644
--- a/drivers/spi/ti_qspi.c
+++ b/drivers/spi/ti_qspi.c
@@ -574,8 +574,8 @@
 		return NULL;
 	}
 
-	cell = fdt_getprop(gd->fdt_blob, bus->of_offset, "syscon-chipselects",
-			   &len);
+	cell = fdt_getprop(gd->fdt_blob, dev_of_offset(bus),
+			   "syscon-chipselects", &len);
 	if (len < 2*sizeof(fdt32_t)) {
 		debug("%s: offset not available\n", __func__);
 		return NULL;