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/arch/x86/cpu/ivybridge/sata.c b/arch/x86/cpu/ivybridge/sata.c
index c7ffbc1..0f5e190 100644
--- a/arch/x86/cpu/ivybridge/sata.c
+++ b/arch/x86/cpu/ivybridge/sata.c
@@ -39,7 +39,7 @@
 {
 	unsigned int port_map, speed_support, port_tx;
 	const void *blob = gd->fdt_blob;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 	const char *mode;
 	u32 reg32;
 	u16 reg16;
@@ -190,7 +190,7 @@
 static void bd82x6x_sata_enable(struct udevice *dev)
 {
 	const void *blob = gd->fdt_blob;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 	unsigned port_map;
 	const char *mode;
 	u16 map = 0;