x86: fix cpufreq + sched_clock() regression

I noticed that my sched_clock() was slow on a number of machine, so I
started looking at cpufreq.

The below seems to fix the problem for me.
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/tsc.c

@@ -314,7 +314,7 @@ static int time_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
 			mark_tsc_unstable("cpufreq changes");
 	}
 
-	set_cyc2ns_scale(tsc_khz_ref, freq->cpu);
+	set_cyc2ns_scale(tsc_khz, freq->cpu);
 
 	return 0;
 }