iio: cros_ec: Add kernel-doc for cros_ec_sensors_read_lpc

Document cros_ec_sensors_read_lpc, adding an additional note to explain
that this is the safe function for reading the EC data.

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