iio: sx9324: Add setting for CS idle mode

Based on device tree setting, set the CS idle mode.

Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20220503193937.3794477-3-swboyd@chromium.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>