iio: cros_ec_accel_legacy: Use cros_ec_sensors_core

Remove duplicate code in cros-ec-accel-legacy,
use cros-ec-sensors-core functions and structures when possible.

On glimmer, check the 2 accelerometers are presented and working.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>