hwrng: ks-sa - access private data via struct hwrng

This driver uses a struct ks_sa_rng for its private data. It contains a
struct hwrng. Call container_of to get from hwrng to ks_sa_rng.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>