The multi buffer concurrent requests ahash speed test only
supported the cycles mode. Add support for the so called
jiffies mode that test performance of bytes/sec.

We only add support for digest mode at the moment.
Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>