crypto: sahara - do not resize req->src when doing hash operations

When testing sahara sha256 speed performance with tcrypt (mode=404) on
imx53-qsrb board, multiple "Invalid numbers of src SG." errors are
reported. This was traced to sahara_walk_and_recalc() resizing req->src
and causing the subsequent dma_map_sg() call to fail.

Now that the previous commit fixed sahara_sha_hw_links_create() to take
into account the actual request size, rather than relying on sg->length
values, the resize operation is no longer necessary.

Therefore, remove sahara_walk_and_recalc() and simplify associated logic.

Fixes: 5a2bb93f ("crypto: sahara - add support for SHA1/256")
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>