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/net/cpsw-common.c b/drivers/net/cpsw-common.c
index e828e85..aa52503 100644
--- a/drivers/net/cpsw-common.c
+++ b/drivers/net/cpsw-common.c
@@ -20,7 +20,7 @@
 				       int slave, u8 *mac_addr)
 {
 	void *fdt = (void *)gd->fdt_blob;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 	u32 macid_lsb;
 	u32 macid_msb;
 	fdt32_t gmii = 0;
@@ -60,7 +60,7 @@
 				    u8 *mac_addr)
 {
 	void *fdt = (void *)gd->fdt_blob;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 	u32 macid_lo;
 	u32 macid_hi;
 	fdt32_t gmii = 0;