tools/power turbostat: Fix Core C6 residency on Atom CPUs

For Atom CPUs that have core cstate deeper than C6,
MSR_CORE_C6_RESIDENCY actually returns the residency for both CC6 and
deeper Core cstates.
Thus, the real Core C6 residency should be the subtraction of
MSR_CORE_C6_RESIDENCY return value and MSR_CORE_C6_RESIDENCY return value.

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