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: 70fe238594 ("fdt: Allow the devicetree to come from a bloblist")

Signed-off-by: Simon Glass <sjg@chromium.org>

lib/fdtdec.c

@@ -1685,6 +1685,7 @@ int fdtdec_setup(void)
 				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;