regulator: Fix setting low power mode for wm831x aldo

Set BIT[8] of LDO7 ON Control mode for low power mode.

R16507 (407Bh)  LDO7 ON Control
BIT[8] LDO7_ON_MODE: LDO7 ON Operating Mode
        0 = Normal mode
        1 = Low Power mode

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>