crypto: authenc - Consider ahash ASYNC bit

As it is, if you get an async ahash with a sync skcipher you'll
end up with a sync authenc, which is wrong.

This patch fixes it by considering the ASYNC bit from ahash as
well.

It also fixes a little bug where if a sync version of authenc
is requested we may still end up using an async ahash.

Neither of them should have any effect as none of the authenc
users can request for a sync authenc.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>