drivers/rn2xx3: fix incorrectly terminated hex strings
drivers/rn2xx3: fix incorrectly terminated hex strings

When writing an array of bytes, each byte is converted on the fly to 2 hex characters and sent to the rn2xx3 module. This change ensures each string is correctly terminated, with null character, when written to the module

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