crypto: rng - Ensure set_ent is always present

Ensure that set_ent is always set since only drbg provides it.

Fixes: 77ebdabe ("crypto: af_alg - add extra parameters for DRBG interface")
Reported-by: Yiqi Sun <sunyiqixm@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>