crypto: echainiv - Fix encryption convention

This patch fixes a bug where we were incorrectly including the
IV in the AD during encryption.  The IV must remain in the plain
text for it to be encrypted.

During decryption there is no need to copy the IV to dst because
it's now part of the AD.

This patch removes an unncessary check on authsize which would be
performed by the underlying decrypt call.

Finally this patch makes use of the type-safe init/exit functions.

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