regulator: twl6030: do not write to _GRP for regulator disable

TWL6030: regulator is disabled via VREG_STATE
TWL4030: regulator is disabled via VREG_GRP
Since there is nothing common, split twlreg_enable similar to other regulator_ops

Signed-off-by: Balaji T K <balajitk@ti.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>