gnrc_netif: fix DEBUG output on error
gnrc_netif: fix DEBUG output on error

`res` is an integer that is always negative in the given case, so using
`%u` here might be a little bit confusing.

x3ro/RIOT,kaspar030/RIOT,mfrey/RIOT,toonst/RIOT,aeneby/RIOT,smlng/RIOT,kaspar030/RIOT,authmillenon/RIOT,OlegHahm/RIOT,yogo1212/RIOT,OTAkeys/RIOT,rfuentess/RIOT,kYc0o/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,cladmi/RIOT,aeneby/RIOT,ant9000/RIOT,RIOT-OS/RIOT,mtausig/RIOT,miri64/RIOT,yogo1212/RIOT,kaspar030/RIOT,kYc0o/RIOT,authmillenon/RIOT,mfrey/RIOT,aeneby/RIOT,smlng/RIOT,rfuentess/RIOT,josephnoir/RIOT,yogo1212/RIOT,rfuentess/RIOT,jasonatran/RIOT,rfuentess/RIOT,yogo1212/RIOT,lazytech-org/RIOT,ant9000/RIOT,basilfx/RIOT,RIOT-OS/RIOT,miri64/RIOT,x3ro/RIOT,toonst/RIOT,smlng/RIOT,cladmi/RIOT,OTAkeys/RIOT,lazytech-org/RIOT,josephnoir/RIOT,OlegHahm/RIOT,aeneby/RIOT,OlegHahm/RIOT,jasonatran/RIOT,ant9000/RIOT,jasonatran/RIOT,mtausig/RIOT,mfrey/RIOT,ant9000/RIOT,RIOT-OS/RIOT,toonst/RIOT,OlegHahm/RIOT,jasonatran/RIOT,josephnoir/RIOT,yogo1212/RIOT,basilfx/RIOT,mtausig/RIOT,cladmi/RIOT,smlng/RIOT,kYc0o/RIOT,jasonatran/RIOT,basilfx/RIOT,lazytech-org/RIOT,mfrey/RIOT,authmillenon/RIOT,josephnoir/RIOT,x3ro/RIOT,kYc0o/RIOT,kaspar030/RIOT,toonst/RIOT,miri64/RIOT,kaspar030/RIOT,mtausig/RIOT,basilfx/RIOT,kYc0o/RIOT,authmillenon/RIOT,lazytech-org/RIOT,miri64/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,yogo1212/RIOT,cladmi/RIOT,RIOT-OS/RIOT,cladmi/RIOT,x3ro/RIOT,mfrey/RIOT,basilfx/RIOT,smlng/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,miri64/RIOT,toonst/RIOT,aeneby/RIOT,ant9000/RIOT,rfuentess/RIOT,x3ro/RIOT,mtausig/RIOT,authmillenon/RIOT,josephnoir/RIOT,lazytech-org/RIOT