crypto: sun8i-ss - handle zero sized sg

sun8i-ss does not handle well the possible zero sized sg.

Fixes: d9b45418

 ("crypto: sun8i-ss - support hash algorithms")
Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>