s390x: Fix HMAC digest detection

Use EVP_MD_is_a() instead of EVP_MD_get_type() to detect the digest
type. EVP_MD_get_type() does not always return the expected NID, e.g.
when running in the FIPS provider, EVP_MD_get_type() returns zero,
causing to skip the HMAC acceleration path.

Signed-off-by: Ingo Franzki <ifranzki@linux.ibm.com>

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25304)