crypto: sahara - fix ahash reqsize

Set the reqsize for sha algorithms to sizeof(struct sahara_sha_reqctx), the
extra space is not needed.

Fixes: 5a2bb93f

 ("crypto: sahara - add support for SHA1/256")
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>