crypto: qat - implement dh fallback for primes > 4K

The Intel QAT driver provides support for the Diffie-Hellman (DH)
algorithm, limited to prime numbers up to 4K. This driver is used
by default on platforms with integrated QAT hardware for all DH requests.
This has led to failures with algorithms requiring larger prime sizes,
such as ffdhe6144.

  alg: ffdhe6144(dh): test failed on vector 1, err=-22
  alg: self-tests for ffdhe6144(qat-dh) (ffdhe6144(dh)) failed (rc=-22)

Implement a fallback mechanism when an unsupported request is received.

Signed-off-by: Damian Muszynski <damian.muszynski@intel.com>
Reviewed-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>