crypto: caam - trivial code clean-up

-replace offsetof with container_of
-remove unused "assoc_nents", "iv_dma" from aead_edesc
and fix comments
-remove unused CAAM_MAX_IV_LENGTH #define

Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>