drivers/adcxx1c: fix SAUL return value in case of read error
drivers/adcxx1c: fix SAUL return value in case of read error

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