crypto: sun8i-ss - use sg_nents_for_len

When testing with some large SG list, the sun8i-ss drivers always
fallback even if it can handle it.
So use sg_nents_for_len() which permits to see less SGs than needed.

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