crypto: ixp4xx - Fix handling of chained sg buffers

 - keep dma functions away from chained scatterlists.
   Use the existing scatterlist iteration inside the driver
   to call dma_map_single() for each chunk and avoid dma_map_sg().

Signed-off-by: Christian Hohnstaedt <chohnstaedt@innominate.com>
Tested-By: Karl Hiramoto <karl@hiramoto.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>