iio: cros_ec_accel_legacy: Fix incorrect channel setting

INFO_SCALE is set both for each channel and all channels.
iio is using all channel setting, so the error was not user visible.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>