cpupower: Do not change the frequency of offline cpu

Check if the cpu is online before changing the frequency/governor of
the cpu.

Reported-by: Pavaman Subramaniyam <pavsubra@linux.vnet.ibm.com>
Signed-off-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Acked-by: Thomas Renninger <trenn@suse.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>