drivers/dht: Worked around doxygen bug
drivers/dht: Worked around doxygen bug

Markdown in @retval values is currently broken in Doxygen. This removes use
of markdown there

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