tests/pkg_semtech_loramac: fix get command for EUIs
tests/pkg_semtech_loramac: fix get command for EUIs

- add a null character after conversion from bytes to hex chars
- set the maximum print buffer size to the maximum possible size (application key size)

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