hwrng: timeriomem - Fix cooldown period calculation

Ensure cooldown period tolerance of 1% is actually accounted for.

Fixes: ca3bff70

 ("hwrng: timeriomem - Improve performance...")
Signed-off-by: Jan Henrik Weinstock <jan.weinstock@rwth-aachen.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>