hwrng: core - zeroize buffers with random data

The HWRNG core allocates two buffers during initialization which are
used to obtain random data. After that data is processed, it is now
zeroized as it is possible that the HWRNG core will not be asked to
produce more random data for a long time. This prevents leaving such
sensitive data in memory.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>