tools/power turbostat: correctly decode of ENERGY_PERFORMANCE_BIAS

When EPB is 0xF, turbosat was incorrectly describing it as "custom"
instead of calling it "powersave":

< cpu0: MSR_IA32_ENERGY_PERF_BIAS: 0x0000000f (custom)
> cpu0: MSR_IA32_ENERGY_PERF_BIAS: 0x0000000f (powersave)

Signed-off-by: Len Brown <len.brown@intel.com>