PM / OPP: Make local function ti_opp_supply_set_opp() static

Fixes the following sparse warning:

drivers/opp/ti-opp-supply.c:276:5: warning:
 symbol 'ti_opp_supply_set_opp' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>