tools/power turbostat: rename tcc variables

There are two TCC activation temeprature.
One is the default TCC activation temperature, also known as TJ_MAX.
Another one is the effective TCC activation temperature, which is the
subtraction of default TCC activation temperature and TCC offset.

The name of variable tcc_activation_temp might be misleading here.
Thus rename tcc_activation_temp to tj_max, and use tcc_default and
tcc_offset to calculate the effective TCC activation temperature.

No functional change in this patch.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>