cpufreq: powernv: Fix bugs in powernv_cpufreq_{init/exit}

Unregister the notifiers if cpufreq_driver_register() fails in
powernv_cpufreq_init(). Re-arrange the unregistration and cleanup routines
in powernv_cpufreq_exit() to free all the resources after the driver
has unregistered.

Signed-off-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>