crypto: sahara - replace sahara_sg_length with sg_nents_for_len

The sahara_sg_length function of the sahara driver is the same
as sg_nents_for_len from lib/scatterlist.c

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