tools/power/x86/intel-speed-select: cpufreq reads on offline CPUs

Due to some recent kernel changes, reading cpufreq attributes like
scaling_max_freq on offline CPUs returns error. So avoid reading
cpufreq attributes on offline CPUs.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>