OPP: Reuse _opp_compare_key() in _opp_add_static_v2()

Reuse _opp_compare_key() in _opp_add_static_v2() instead of just
comparing frequency while finding suspend frequency. Also add a comment
over _opp_compare_key() explaining its return values.

Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>