cpufreq: governor: rename cur_policy as policy

Just call it 'policy', cur_policy is unnecessarily long and doesn't
have any special meaning.

Reviewed-by: Preeti U Murthy <preeti@linux.vnet.ibm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>