regulator: palmas: Clear PALMAS_SMPS12_CTRL_MODE_ACTIVE_MASK bits in palmas_set_mode_smps

Current code actually clears more bits than PALMAS_SMPS12_CTRL_MODE_ACTIVE_MASK bits.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Graeme Gregory <gg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>