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/pic32_eth.c b/drivers/net/pic32_eth.c
index 167af8b..385aad5 100644
--- a/drivers/net/pic32_eth.c
+++ b/drivers/net/pic32_eth.c
@@ -535,7 +535,8 @@
 	int offset = 0;
 	int phy_addr = -1;
 
-	addr = fdtdec_get_addr_size(gd->fdt_blob, dev->of_offset, "reg", &size);
+	addr = fdtdec_get_addr_size(gd->fdt_blob, dev_of_offset(dev), "reg",
+				    &size);
 	if (addr == FDT_ADDR_T_NONE)
 		return -EINVAL;
 
@@ -544,7 +545,8 @@
 
 	/* get phy mode */
 	pdata->phy_interface = -1;
-	phy_mode = fdt_getprop(gd->fdt_blob, dev->of_offset, "phy-mode", NULL);
+	phy_mode = fdt_getprop(gd->fdt_blob, dev_of_offset(dev), "phy-mode",
+			       NULL);
 	if (phy_mode)
 		pdata->phy_interface = phy_get_interface_by_name(phy_mode);
 	if (pdata->phy_interface == -1) {
@@ -553,13 +555,13 @@
 	}
 
 	/* get phy addr */
-	offset = fdtdec_lookup_phandle(gd->fdt_blob, dev->of_offset,
+	offset = fdtdec_lookup_phandle(gd->fdt_blob, dev_of_offset(dev),
 				       "phy-handle");
 	if (offset > 0)
 		phy_addr = fdtdec_get_int(gd->fdt_blob, offset, "reg", -1);
 
 	/* phy reset gpio */
-	gpio_request_by_name_nodev(gd->fdt_blob, dev->of_offset,
+	gpio_request_by_name_nodev(gd->fdt_blob, dev_of_offset(dev),
 				   "reset-gpios", 0,
 				   &priv->rst_gpio, GPIOD_IS_OUT);