Avoid unnecessary SWAP in cryptsha512-opencl.

- 8% gain on GTX 570 (now it runs at 14.2K c/s)
- Also, HD 6770 (driver 14.6beta) achieve the original performance (from 12.6 driver era)
- jtrTestSuite ok on bull

I had problems with git, merge with care.
Please, check this using your hardware.