drivers/ata8520e: fix potentially unused variables
drivers/ata8520e: fix potentially unused variables

This was reported by scan-build

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