crypto: crypto4xx - avoid VLA use

This patch fixes some of the -Wvla warnings.

crypto4xx_alg.c:83:19: warning: Variable length array is used.
crypto4xx_alg.c:273:56: warning: Variable length array is used.
crypto4xx_alg.c:380:32: warning: Variable length array is used.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>