netdev: add funcs for declaring get/set as notsup
netdev: add funcs for declaring get/set as notsup

This commit adds two convenience functions that allow to declare
get() and/or set() as not supported when declaring a
netdev_driver struct.

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