crypto: caam - check return value of sg_nents_for_len

The sg_nents_for_len() function could fail, this patch add a check for
its return value.
We do the same for sg_count since it use sg_nents_for_len().

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