opp: Allow _set_opp() to work for non-freq devices

The _set_opp() helper will be used for devices which don't change frequency
(like power domains, etc.) later on, prepare for that by not relying on
frequency for making decisions here.

While at it, also update the debug print to contain all relevant
information.

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