crypto: sahara - fix error handling in sahara_hw_descriptor_create()

Do not call dma_unmap_sg() for scatterlists that were not mapped
successfully.

Fixes: 5de88752

 ("crypto: sahara - Add driver for SAHARA2 accelerator.")
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>