crypto: talitos - Do not modify req->cryptlen on decryption.

For decrypt, req->cryptlen includes the size of the authentication
part while all functions of the driver expect cryptlen to be
the size of the encrypted data.

As it is not expected to change req->cryptlen, this patch
implements local calculation of cryptlen.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Fixes: 9c4a7965 ("crypto: talitos - Freescale integrated security engine (SEC) driver")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>