OPP: Reuse dev_pm_opp_get_freq_indexed()

Reuse dev_pm_opp_get_freq_indexed() from dev_pm_opp_get_freq().

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>