more Maxwell (GM204) tuning + minor fix

Minor tuning to get additional 9% speedup on GM204 (for the no-hash
version).
- max register used: 47
- back to 256 threads per block
- also fix a minor compile error bug the non-chance templated path.

Pos 2, perft 7 on GTX970 @ 1500/3700 MHz is now 50 billion moves per
second!
GPU Perft 7: 374190009323,   Time taken: 7.4711 seconds, nps:
50085001317