crypto: sun4i-ss - use crypto_ahash_digestsize

The size of the digest is different between MD5 and SHA1 so instead of
using the higher value (5 words), let's use crypto_ahash_digestsize().

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>