tools/power turbostat: work around RC6 counter wrap

Sometimes the rc6 sysfs counter spontaneously resets,
causing turbostat prints a very large number
as it tries to calcuate % = 100 * (old - new) / interval

When we see (old > new), print ***.**% instead
of a bogus huge number.

Note that this detection is not fool-proof, as the counter
could reset several times and still result in new > old.

Signed-off-by: Len Brown <len.brown@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>