crypto: sahara - dma_map_sg can handle chained SG

The sahara driver use two dma_map_sg path according to SG are chained
or not.
Since dma_map_sg can handle both case, clean the code with all
references to sg chained.

Thus removing the sahara_sha_unmap_sg function.

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