regulator: Fix restoring pmic.dcdcx_hib_mode settings in wm8350_dcdc_set_suspend_enable

What we want is to restore wm8350->pmic.dcdcx_hib_mode settings to
WM8350_DCDCx_LOW_POWER registers. Current code also clears all other
bits of WM8350_DCDCx_LOW_POWER registers which is wrong.

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