dm: core: Correct the return value for uclass_find_first_device()

This function returns -ENODEV when there is no device. This is
inconsistent with other functions, such as uclass_find_next_device(),
which returns 0.

Update it and tidy up the incorrect '-1' values in the comments.

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

drivers/core/uclass.c

@@ -225,7 +225,7 @@ int uclass_find_first_device(enum uclass_id id, struct udevice **devp)
 	if (ret)
 		return ret;
 	if (list_empty(&uc->dev_head))
-		return -ENODEV;
+		return 0;
 
 	*devp = list_first_entry(&uc->dev_head, struct udevice, uclass_node);