opp: ti-opp-supply: Correct the supply in _get_optimal_vdd_voltage call

_get_optimal_vdd_voltage call provides new_supply_vbb->u_volt
as the reference voltage while it should be really new_supply_vdd->u_volt.

Cc: 4.16+ <stable@vger.kernel.org> # v4.16+
Fixes: 9a835fa6

 ("PM / OPP: Add ti-opp-supply driver")
Signed-off-by: Keerthy <j-keerthy@ti.com>
Acked-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>