cmd: fdt: Correct checking of configuration node

fit_conf_get_node() returns a negative value on error.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/fdt.c b/cmd/fdt.c
index aae3278..2401ea8 100644
--- a/cmd/fdt.c
+++ b/cmd/fdt.c
@@ -733,7 +733,7 @@
 
 		gd->fdt_blob = blob;
 		cfg_noffset = fit_conf_get_node(working_fdt, NULL);
-		if (!cfg_noffset) {
+		if (cfg_noffset < 0) {
 			printf("Could not find configuration node: %s\n",
 			       fdt_strerror(cfg_noffset));
 			return CMD_RET_FAILURE;