OPP: Remove rate_not_available parameter to _opp_add()

commit 32715be4 ("opp: Fix adding OPP entries in a wrong order if
rate is unavailable") removed the only user of this field, get rid of
rest of it now.

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