scanner: Remove return doxygen annotation

Replace \returns with just Returns, as this is not a doxygen comment block.
(Avoids differing with the existing convention of \return for return values.)

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>