dm: core: Correct bug introduced in uclass_first/next_device()

These functions now rely on uclass_find_first/next_device() and assume that
they will either return failure (-ve error code) or a device. In fact,
coming to the end of a list is not considered failure and they return 0
in that case.

The logic to deal with this was replaced in commit acb9ca2a with just using
uclass_get_device_tail(). Add back the missing logic. This bug was
caught by unit tests but since they were broken for other reasons at the
time, this was not noticed.

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

drivers/core/uclass.c

@@ -277,6 +277,7 @@ int uclass_get_device_tail(struct udevice *dev, int ret,
 	if (ret)
 		return ret;
 
+	assert(dev);
 	ret = device_probe(dev);
 	if (ret)
 		return ret;
@@ -342,6 +343,8 @@ int uclass_first_device(enum uclass_id id, struct udevice **devp)
 
 	*devp = NULL;
 	ret = uclass_find_first_device(id, &dev);
+	if (!dev)
+		return 0;
 	return uclass_get_device_tail(dev, ret, devp);
 }
 
@@ -352,6 +355,8 @@ int uclass_next_device(struct udevice **devp)
 
 	*devp = NULL;
 	ret = uclass_find_next_device(&dev);
+	if (!dev)
+		return 0;
 	return uclass_get_device_tail(dev, ret, devp);
 }