Put difficilty multiplication back in place.

I removed that some time ago
(https://github.com/MaxDZ8/sph-sgminer/commit/3eaea804e29a4e51e9c742f093c8b00700b3a58b)
and a note on the commit explicitly claims the miner to work... but it
didn't. I probably missed some other change.
Gets some accepts, followed by duplicate shares, but that's sufficient
to test validity.