crypto: caam - set DK (Decrypt Key) bit only for AES accelerator

AES currently shares descriptor creation functions with DES and 3DES.
DK bit is set in all cases, however it is valid only for
the AES accelerator.

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