crypto: amcc - replace get_sg_count by sg_nents_for_len

The get_sg_count function of amcc 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>