cpufreq: return early from __cpufreq_driver_getavg()

There is no need to do cpufreq_get_cpu() and cpufreq_put_cpu() for drivers that
don't support getavg() routine.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Blechd0se <alex.christ@hotmail.de>
Signed-off-by: renderbroken <zkennedy87@gmail.com>

Conflicts:
	drivers/cpufreq/cpufreq.c