regulator: wm8350-regulator - fix the logic of checking REGULATOR_MODE_STANDBY mode

In wm8350_dcdc_set_mode(), we set DCx_SLEEP bit of WM8350_DCDC_SLEEP_OPTIONS
register for REGULATOR_MODE_STANDBY mode.
( DCx_SLEEP bits: 0: Normal DC-DC operation 1: Select LDO mode )

In wm8350_dcdc_get_mode(), current logic to determinate
REGULATOR_MODE_STANDBY mode is just reverse.
( sleep is set should mean 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>