opp: Split _set_opp() out of dev_pm_opp_set_rate()

The _set_opp() helper will be used for devices which don't change their
frequency (like power domains, etc.) later on, prepare for that by
breaking the generic part out of dev_pm_opp_set_rate().

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Dmitry Osipenko <digetx@gmail.com>