iio: cros_ec_sensors: Fix return value to get raw and calibbias data.

The cros_ec_sensors_read function must return the type of value on all
cases. This was always true except for RAW and CALIBBIAS data which
returned an error or 0. This patch just fixes the mistake I introduced
when submitting the series.

Fixes: commit c14dca07 (iio: cros_ec_sensors: add ChromeOS EC
Contiguous Sensors driver)

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>