FFC cleanups

FFC cleanups

Discovered during coverage testing.

Remove unneccesary check when using ossl_dh_get0_params() and
ossl_dsa_get0_params(). These point to addresses and can not fail
for any existing calls.

Make dsa keygen tests only available in the FIPS module - as they are
not used in the default provider.

Change ossl_ffc_set_digest() to return void as it cannot fail.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20359)