hwrng: core - Changed from using strncat to strlcat

The buf is used to hold the list of hwrng devices registered.
The old code ensures we don't walk off the end of buf as we
fill it, but it's unnecessarily complicated and thus difficult
to maintain. Simplify it by using strlcat.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Reviewed-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>