regulator: wm831x-ldo - fix the logic to set REGULATOR_MODE_IDLE and REGULATOR_MODE_STANDBY modes

Problem description in current implementation:
When setting REGULATOR_MODE_IDLE mode, current implementation set
WM831X_LDO1_LP_MODE bit of ctrl_reg (which is wrong, it should clear the bit).
But due to a missing break statement for case REGULATOR_MODE_IDLE, the code
fall through to case REGULATOR_MODE_STANDBY and then clear
WM831X_LDO1_LP_MODE bit. So it still looks OK when checking the status
by wm831x_gp_ldo_get_mode().

When setting REGULATOR_MODE_STANDBY mode, it just does not work.
wm831x_gp_ldo_get_mode() will still return REGULATOR_MODE_IDLE because
the accordingly WM831X_LDO1_LP_MODE bit is clear.

Correct behavior should be:
Clear WM831X_LDO1_LP_MODE bit of ctrl_reg for REGULATOR_MODE_IDLE mode.
Set WM831X_LDO1_LP_MODE bit of ctrl_reg for REGULATOR_MODE_STANDBY mode.

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