Here it would be nice filter the coupon code with [`woocommerce_coupon_code`](https://github.com/woothemes/woocommerce/blob/master/includes/wc-core-functions.php#L32-L33) before.
With the filter, you no longer need [`wc_clean()`](https://github.com/woothemes/woocommerce/blob/master/includes/wc-formatting-functions.php#L241-L250).