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 <[e-mail]>
Signed-off-by: Herbert Xu <[e-mail]>

drivers/char/hw_random/timeriomem-rng.c

···
 		priv->rng_ops.quality = pdata->quality;
 	}
 
-	priv->period = ns_to_ktime(period * NSEC_PER_USEC);
+	priv->period = us_to_ktime(period);
 	init_completion(&priv->completion);
 	hrtimer_setup(&priv->timer, timeriomem_rng_trigger, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);