cpupower: Correct return type of cpu_power_is_cpu_online() in cpufreq-set

When converting to a shared library in ac5a181d ("cpupower: Add
cpuidle parts into library"), cpu_freq_cpu_exists() was converted to
cpupower_is_cpu_online(). cpu_req_cpu_exists() returned 0 on success and
-ENOSYS on failure whereas cpupower_is_cpu_online returns 1 on success.
Check for the correct return value in cpufreq-set.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=1374212
Fixes: ac5a181d

 (cpupower: Add cpuidle parts into library)
Reported-by: Julian Seward <jseward@acm.org>
Signed-off-by: Laura Abbott <labbott@redhat.com>
Acked-by: Thomas Renninger <trenn@suse.com>
Cc: 4.7+ <stable@vger.kernel.org> # 4.7+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>