crypto: picoxcell - 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.
In the same time, we remove sg_count() as it is used as an alias of
sg_nents_for_len.

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