opp: Allow disabled OPPs in dev_pm_opp_get_freq()

Allow dev_pm_opp_get_freq() to work for disabled OPPs.

Signed-off-by: Andrew-sh.Cheng <andrew-sh.cheng@mediatek.com>
[ Viresh: Massaged commit log ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>