image-fdt.c: store returned error value

This fixes the following warning (and the runtime error reporting):
../common/image-fdt.c:491:4: warning: 'fdt_ret' may be used
uninitialized in this function [-Wmaybe-uninitialized]
Signed-off-by: NMax Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: NMarcel Ziswiler <marcel.ziswiler@toradex.com>
Acked-by: NMarek Vasut <marex@denx.de>
Acked-by: NSimon Glass <sjg@chromium.org>

common/image-fdt.c

@@ -492,7 +492,8 @@ int image_setup_libfdt(bootm_headers_t *images, void *blob,
 		}
 	}
 	if (IMAGE_OF_SYSTEM_SETUP) {
-		if (ft_system_setup(blob, gd->bd)) {
+		fdt_ret = ft_system_setup(blob, gd->bd);
+		if (fdt_ret) {
 			printf("ERROR: system-specific fdt fixup failed: %s\n",
 			       fdt_strerror(fdt_ret));
 			goto err;