tools/power turbostat: set max_num_cpus equal to the cpumask length

Future fixes will use sysfs files that contain cpumask output.  The code
needs to know the length of the cpumask in order to determine which cpus
are set in a cpumask.  Currently topo.max_cpu_num is the maximum cpu
number.  It can be increased the the maximum value of cpus represented in
cpumasks.

Set max_num_cpus to the length of a cpumask.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Len Brown <len.brown@intel.com>