driver/slipdev: ignore implicit fallthrough error by GCC 7
driver/slipdev: ignore implicit fallthrough error by GCC 7

rfuentess/RIOT,authmillenon/RIOT,miri64/RIOT,rfuentess/RIOT,OlegHahm/RIOT,BytesGalore/RIOT,LudwigOrtmann/RIOT,kaspar030/RIOT,ant9000/RIOT,mtausig/RIOT,A-Paul/RIOT,gebart/RIOT,x3ro/RIOT,mfrey/RIOT,gebart/RIOT,avmelnikoff/RIOT,toonst/RIOT,mfrey/RIOT,lazytech-org/RIOT,RIOT-OS/RIOT,A-Paul/RIOT,mfrey/RIOT,biboc/RIOT,kbumsik/RIOT,aeneby/RIOT,x3ro/RIOT,LudwigKnuepfer/RIOT,avmelnikoff/RIOT,A-Paul/RIOT,smlng/RIOT,kYc0o/RIOT,toonst/RIOT,basilfx/RIOT,ant9000/RIOT,neiljay/RIOT,biboc/RIOT,toonst/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,yogo1212/RIOT,smlng/RIOT,aeneby/RIOT,miri64/RIOT,OlegHahm/RIOT,lazytech-org/RIOT,mfrey/RIOT,neiljay/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,rfuentess/RIOT,mtausig/RIOT,basilfx/RIOT,OTAkeys/RIOT,A-Paul/RIOT,LudwigOrtmann/RIOT,kaspar030/RIOT,josephnoir/RIOT,kaspar030/RIOT,yogo1212/RIOT,lazytech-org/RIOT,x3ro/RIOT,BytesGalore/RIOT,gebart/RIOT,gebart/RIOT,miri64/RIOT,A-Paul/RIOT,RIOT-OS/RIOT,miri64/RIOT,LudwigKnuepfer/RIOT,BytesGalore/RIOT,mtausig/RIOT,jasonatran/RIOT,basilfx/RIOT,ant9000/RIOT,BytesGalore/RIOT,authmillenon/RIOT,LudwigOrtmann/RIOT,neiljay/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,smlng/RIOT,BytesGalore/RIOT,kYc0o/RIOT,rfuentess/RIOT,cladmi/RIOT,mfrey/RIOT,jasonatran/RIOT,x3ro/RIOT,kbumsik/RIOT,cladmi/RIOT,josephnoir/RIOT,neiljay/RIOT,aeneby/RIOT,biboc/RIOT,OlegHahm/RIOT,rfuentess/RIOT,avmelnikoff/RIOT,kbumsik/RIOT,OTAkeys/RIOT,lazytech-org/RIOT,OTAkeys/RIOT,smlng/RIOT,LudwigOrtmann/RIOT,authmillenon/RIOT,authmillenon/RIOT,mtausig/RIOT,gebart/RIOT,basilfx/RIOT,kaspar030/RIOT,toonst/RIOT,ant9000/RIOT,yogo1212/RIOT,kYc0o/RIOT,jasonatran/RIOT,yogo1212/RIOT,LudwigKnuepfer/RIOT,LudwigOrtmann/RIOT,josephnoir/RIOT,kbumsik/RIOT,basilfx/RIOT,avmelnikoff/RIOT,cladmi/RIOT,lazytech-org/RIOT,kaspar030/RIOT,biboc/RIOT,jasonatran/RIOT,mtausig/RIOT,LudwigKnuepfer/RIOT,toonst/RIOT,kYc0o/RIOT,OTAkeys/RIOT,LudwigOrtmann/RIOT,LudwigKnuepfer/RIOT,aeneby/RIOT,avmelnikoff/RIOT,neiljay/RIOT,yogo1212/RIOT,smlng/RIOT,yogo1212/RIOT,kbumsik/RIOT,x3ro/RIOT,biboc/RIOT,miri64/RIOT,authmillenon/RIOT,authmillenon/RIOT,cladmi/RIOT,aeneby/RIOT,josephnoir/RIOT,jasonatran/RIOT,OlegHahm/RIOT,cladmi/RIOT,OlegHahm/RIOT,ant9000/RIOT