crypto: talitos - do not perform unnecessary dma synchronisation

req_ctx->hw_context is mainly used only by the HW. So it is not needed
to sync the HW and the CPU each time hw_context in DMA mapped.
This patch modifies the DMA mapping in order to limit synchronisation
to necessary situations.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>