iio: cros_ec: Remove replacing error code with -EIO

Due to an API misread, error code can be different for -EIO when reading
a sysfs entry. Return the error reported by the cros_ec stack.

Check the proper error message (protocol error, not supported) is
reported when there is an error returned by the EC stack.

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