hwrng: timeriomem - Use us_to_ktime() where appropriate

It is better to replace ns_to_ktime() with us_to_ktime(),
which can make the code clearer.

Signed-off-by: Xichao Zhao <zhao.xichao@vivo.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>