crypto: sahara - fix processing hash requests with req->nbytes < sg->length

It's not always the case that the entire sg entry needs to be processed.
Currently, when nbytes is less than sg->length, "Descriptor length" errors
are encountered.

To fix this, take the actual request size into account when populating the
hw links.

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>