pkg/lwip: fix handling of netdev send return code
pkg/lwip: fix handling of netdev send return code

kaspar030/RIOT,ant9000/RIOT,jasonatran/RIOT,kYc0o/RIOT,jasonatran/RIOT,ant9000/RIOT,miri64/RIOT,miri64/RIOT,kaspar030/RIOT,jasonatran/RIOT,kaspar030/RIOT,jasonatran/RIOT,kaspar030/RIOT,miri64/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,miri64/RIOT,jasonatran/RIOT,ant9000/RIOT,kYc0o/RIOT,ant9000/RIOT,miri64/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kYc0o/RIOT,RIOT-OS/RIOT