tools/power turbostat: Handle non-root legacy-uncore sysfs permissions

/sys/devices/system/cpu/intel_uncore_frequency/package_X_die_Y/
may be readable by all, but
/sys/devices/system/cpu/intel_uncore_frequency/package_X_die_Y/current_freq_khz
may be readable only by root.

Non-root turbostat users see complaints in this scenario.

Fail probe of the interface if we can't read current_freq_khz.

Reported-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Original-patch-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>