x86: fix more TSC clock source calibration errors

The previous patch wasn't correctly handling the 'count' variable.  If
a CPU gave bad results on the 1st or 2nd run but good results on the
3rd, it wouldn't do the correct thing.  No idea if any such CPU
exists, but the patch below handles that case by discarding the bad
runs.

If a bad result (too quick, or too slow) occurs on any of the 3 runs
it will be discarded.

Also updated some comments to explain what's going on.

Signed-off-by: Dave Johnson <djohnson@sw.starentnetworks.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>