crypto: caam - use dma_map_sg() return code

dma_map_sg() might coalesce S/G entries, so use the number of S/G
entries returned by it instead of what sg_nents_for_len() initially
returns.

Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>