hwrng: s390 - replace snprintf in show functions with sysfs_emit

coccicheck complains about the use of snprintf() in sysfs show functions.

Fix the following coccicheck warning:
drivers/char/hw_random/s390-trng.c:114:8-16: WARNING: use scnprintf or sprintf.
drivers/char/hw_random/s390-trng.c:122:8-16: WARNING: use scnprintf or sprintf.

Use sysfs_emit instead of scnprintf or sprintf makes more sense.

Signed-off-by: Qing Wang <wangqing@vivo.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>