rng-tools: fix rngd_jitter initialization

rngd daemon may spam the console when using an older version
than 6.11.

Backport patches from https://github.com/nhorman/rng-tools/pull/99/commits



Signed-off-by: Julien Massot <julien.massot@iot.bzh>
Signed-off-by: Steve Sakoman <steve@sakoman.com>