crypto: sun4i_ss_prng - fix return value of sun4i_ss_prng_generate

According to crypto/rng.h generate function should return 0 on success
and < 0 on error.

Fixes: b8ae5c73

 ("crypto: sun4i-ss - support the Security System PRNG")
Signed-off-by: Artem Savkov <artem.savkov@gmail.com>
Acked-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>