tools/power/x86/intel-speed-select: Max CPU count calculation when CPU0 is offline

Currently /sys/devices/system/cpu/cpu0/topology/thread_siblings is used to
get the max CPU count. But when CPU0 is offline, then this file will be
absent.

So add processing so that we can get count from any first CPU in the
system. which is online.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>