Our handling of ciphers with IV trailing the AAD was correct
but overly complicated. Refactor to simplify and possibly
save one DMA burst.

This has the added bonus of behaving the same as the generic
rfc4543 implementation for none compliants inputs where the
IV in the iv field was not the same as the IV in the AAD.

There should be no change in behaviour with correct inputs.
Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>