syscon: Avoid returning a device on failure

If the device cannot be probed, syscon_get_by_driver_data() will still
return a useful value in its devp parameter. Ensure that it returns NULL
instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

drivers/core/syscon-uclass.c

@@ -38,6 +38,7 @@ int syscon_get_by_driver_data(ulong driver_data, struct udevice **devp)
 	struct uclass *uc;
 	int ret;
 
+	*devp = NULL;
 	ret = uclass_get(UCLASS_SYSCON, &uc);
 	if (ret)
 		return ret;