Use a new algorithm for choosing a thread concurrency when none or no shader value is specified for scrypt.

ocl.c

@@ -482,7 +482,10 @@ _clState *initCl(unsigned int gpu, char *name, size_t nameSize)
 			cgpu->lookup_gap = cgpu->opt_lg;
 
 		if (!cgpu->opt_tc) {
-			cgpu->thread_concurrency = cgpu->max_alloc / 32768 / cgpu->lookup_gap;
+			unsigned int sixtyfours;
+
+			sixtyfours =  cgpu->max_alloc / 131072 / 64 - 1;
+			cgpu->thread_concurrency = sixtyfours * 64;
 			if (cgpu->shaders && cgpu->thread_concurrency > cgpu->shaders) {
 				cgpu->thread_concurrency -= cgpu->thread_concurrency % cgpu->shaders;
 				if (cgpu->thread_concurrency > cgpu->shaders * 5)