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>
diff --git a/test/dm/core.c b/test/dm/core.c
index edd55b0..f74c430 100644
--- a/test/dm/core.c
+++ b/test/dm/core.c
@@ -749,8 +749,7 @@
 		ut_assert(dev);
 	}
 
-	ret = uclass_find_first_device(UCLASS_TEST_DUMMY, &dev);
-	ut_assert(ret == -ENODEV);
+	ut_assertok(uclass_find_first_device(UCLASS_TEST_DUMMY, &dev));
 	ut_assert(!dev);
 
 	return 0;