drivers/at: fix invalid function pointer cast.
drivers/at: fix invalid function pointer cast.

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