platform/chrome: cros_kunit_util: add default value for `msg->result`

Add default value for `msg->result` so that it won't be garbage bytes
when the mock list is empty.

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20220720044754.4026295-3-tzungbi@kernel.org