Remove the dedicated GHASH implementation in GCM, and uses the GHASH
digest algorithm instead. This will make GCM uses hardware accelerated
GHASH implementation automatically if available.

ahash instead of shash interface is used, because some hardware
accelerated GHASH implementation needs asynchronous interface.
Signed-off-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>