regulator: gpio-regulator: populate selector from set_voltage

This was missing until now and the underlying
_regulator_do_set_voltage is using this value when calling list_voltage.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>