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/block/dwc_ahci.c b/drivers/block/dwc_ahci.c
index b2e7150..3f839bf 100644
--- a/drivers/block/dwc_ahci.c
+++ b/drivers/block/dwc_ahci.c
@@ -31,9 +31,9 @@
 	struct scsi_platdata *plat = dev_get_platdata(dev);
 	fdt_addr_t addr;
 
-	plat->max_id = fdtdec_get_uint(gd->fdt_blob, dev->of_offset, "max-id",
-				       CONFIG_SYS_SCSI_MAX_SCSI_ID);
-	plat->max_lun = fdtdec_get_uint(gd->fdt_blob, dev->of_offset,
+	plat->max_id = fdtdec_get_uint(gd->fdt_blob, dev_of_offset(dev),
+				       "max-id", CONFIG_SYS_SCSI_MAX_SCSI_ID);
+	plat->max_lun = fdtdec_get_uint(gd->fdt_blob, dev_of_offset(dev),
 					"max-lun", CONFIG_SYS_SCSI_MAX_LUN);
 
 	priv->base = map_physmem(devfdt_get_addr(dev), sizeof(void *),