hwrng: core - convert sysfs sprintf/snprintf family to sysfs_emit

Fix the following coccicheck warning:

drivers/char/hw_random/core.c:399:8-16: WARNING: use scnprintf or sprintf.

Signed-off-by: Zihao Tang <tangzihao1@hisilicon.com>
Signed-off-by: Jay Fang <f.fangjian@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>