crypto: ux500 - Fix the parameter of dma_unmap_sg()

For function dma_unmap_sg(), the <nents> parameter should be number of
elements in the scatterlist prior to the mapping, not after the mapping.
So fix this usage.

Signed-off-by: Xiang Chen <chenxiang66@hisilicon.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>