OPP: Return opp_table from dev_pm_opp_get_opp_table_ref()

For convenience of users, return back the pointer to the opp_table from
dev_pm_opp_get_opp_table_ref(), so they can do:

	opp_table = dev_pm_opp_get_opp_table_ref(tmp_table);

No intentional functional impact.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>