PM / OPP: Make _find_opp_table_unlocked() static

Fixes the following sparse warning:

drivers/base/power/opp/core.c:49:18: warning:
 symbol '_find_opp_table_unlocked' was not declared. Should it be static?
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: NStephen Boyd <sboyd@codeaurora.org>
Acked-by: NViresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/base/power/opp/core.c

@@ -46,7 +46,7 @@ static struct opp_device *_find_opp_dev(const struct device *dev,
 	return NULL;
 }
 
-struct opp_table *_find_opp_table_unlocked(struct device *dev)
+static struct opp_table *_find_opp_table_unlocked(struct device *dev)
 {
 	struct opp_table *opp_table;