Condition was accidentally reversed, so PIT calibration always failed
when PIT was present and always succeeded when PIT was missing, but in
the latter case resulted in absurdly fast clock.
Reported and tested by Vitaly Kuznetsov <vkuznets@redhat.com>
PIT isn't available on some of new hardware including Hyper-V. So
use pmtimer for calibration. Moreover pmtimer calibration is faster, so
use it on coreboor where booting time is important.
Based on patch by Michael Chang.