fdt: Don't overwrite bloblist devicetree

When the devicetree comes from a bloblist, it is currently overwritten
by the appended one, if present. It should be preserved.

Adjust the logic to support this.

Fixes: 70fe2385943 ("fdt: Allow the devicetree to come from a bloblist")

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/fdtdec.c b/lib/fdtdec.c
index 6865f78..5edc8dd 100644
--- a/lib/fdtdec.c
+++ b/lib/fdtdec.c
@@ -1685,6 +1685,7 @@
 				gd->fdt_src = FDTSRC_BLOBLIST;
 				log_debug("Devicetree is in bloblist at %p\n",
 					  gd->fdt_blob);
+				ret = 0;
 			} else {
 				log_debug("No FDT found in bloblist\n");
 				ret = -ENOENT;