crypto: authenc - Use skcipher

This patch converts authenc to use the new skcipher interface as
opposed to ablkcipher.

It also fixes a little bug where if a sync version of authenc
is requested we may still end up using an async ahash.  This should
have no effect as none of the authenc users can request for a
sync authenc.

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