crypto: sahara - Fix error check for dma_map_sg

dma_map_sg return 0 on error, it returns the number of
DMA address segments mapped (this may be shorter
than <nents> passed in if some elements of the scatter/gather
list are physically or virtually adjacent and an IOMMU maps
them with a single entry).

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: linux-crypto@vger.kernel.org
Cc: linux-kernel@vger.kernel.org

Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>