regulator: fan53555: fill set_voltage_time_set callback

Setting the set_voltage_time_sel callback to the standard function
regulator_set_voltage_time_sel enables the regulator to actually honor
ramp-delays when during regulator_set_voltage calls.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Mark Brown <broonie@kernel.org>