iio: cros_ec: Accept -EOPNOTSUPP as 'not supported' error code

A follow-up patch will extend the number of errors reported by
cros_ec_cmd_xfer_status(). Specifically, the function will return
-EOPNOTSUPP if a command is not supported by the EC. Prepare for it.

Cc: Gwendal Grignou <gwendal@chromium.org>
Cc: Yu-Hsuan Hsu <yuhsuan@chromium.org>
Cc: Prashant Malani <pmalani@chromium.org>
Cc: Brian Norris <briannorris@chromium.org>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>