pkg/lwip: fixes the netif handling for atwinc15x0
pkg/lwip: fixes the netif handling for atwinc15x0

With PR #14349 the double stack mode for lwIP was activated. The ATWINC15x0 driver was provided before the changes in PR #14349 but merged after that. Therefore, the changes for the driver were not taken into account anymore and have to be applied accordingly.
kYc0o/RIOT,OTAkeys/RIOT,miri64/RIOT,kYc0o/RIOT,jasonatran/RIOT,jasonatran/RIOT,miri64/RIOT,authmillenon/RIOT,OTAkeys/RIOT,ant9000/RIOT,OlegHahm/RIOT,authmillenon/RIOT,OlegHahm/RIOT,jasonatran/RIOT,ant9000/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,ant9000/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kYc0o/RIOT,kaspar030/RIOT,authmillenon/RIOT,jasonatran/RIOT,jasonatran/RIOT,kaspar030/RIOT,miri64/RIOT,RIOT-OS/RIOT,ant9000/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,miri64/RIOT,ant9000/RIOT,authmillenon/RIOT,kaspar030/RIOT,miri64/RIOT,authmillenon/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,OlegHahm/RIOT