cve-check: fix return type in check_cves

Make empty return types in check_cvs the same for all code paths.

Signed-off-by: Marta Rybczynska <marta.rybczynska@huawei.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit f86393c9)
Signed-off-by: Steve Sakoman <steve@sakoman.com>