twl-regulator: Fix reg_disable functionality for 4030 and 6030

This change makes sure all regulator group assignments are cleared on
disable call

Signed-off-by: Juha Keski-Saari <ext-juha.1.keski-saari@nokia.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>