crypto: chacha20poly1305 - Use skcipher

This patch converts chacha20poly1305 to use the new skcipher
interface as opposed to ablkcipher.

It also fixes a buglet where we may end up with an async poly1305
when the user asks for a async algorithm.  This shouldn't be a
problem yet as there aren't any async implementations of poly1305
out there.

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