RAKP OpenCL tweaks.
Try to fix a regression on 13.4 Catalyst driver (revert performance to 13.1 situation).

 From:
 Local worksize (LWS) 64, global worksize (GWS) 32768
 Benchmarking: sha512crypt-opencl, crypt(3) $6$ (rounds=5000) [SHA512 OpenCL]... DONE
 Raw:    8467 c/s real, 819200 c/s virtual

To:
 Local worksize (LWS) 64, global worksize (GWS) 32768
 Benchmarking: sha512crypt-opencl, crypt(3) $6$ (rounds=5000) [SHA512 OpenCL]... DONE
 Raw:    13484 c/s real, 655360 c/s virtual