cpufreq: Rename __cpufreq_governor() to cpufreq_governor()

The __ at the beginning of the routine aren't really necessary at all.
Rename it to cpufreq_governor() instead.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>