cpu/cc26x0: fix I2C compilation error with NDEBUG
cpu/cc26x0: fix I2C compilation error with NDEBUG

When NDEBUG macro is defined during compilation, the assert macro produces empty code. The dev parameter is then unused.
toonst/RIOT,RIOT-OS/RIOT,miri64/RIOT,jasonatran/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,smlng/RIOT,basilfx/RIOT,miri64/RIOT,miri64/RIOT,jasonatran/RIOT,smlng/RIOT,smlng/RIOT,OlegHahm/RIOT,kaspar030/RIOT,authmillenon/RIOT,toonst/RIOT,kaspar030/RIOT,OTAkeys/RIOT,ant9000/RIOT,kYc0o/RIOT,toonst/RIOT,yogo1212/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,basilfx/RIOT,ant9000/RIOT,OlegHahm/RIOT,yogo1212/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,basilfx/RIOT,kYc0o/RIOT,yogo1212/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,miri64/RIOT,authmillenon/RIOT,toonst/RIOT,authmillenon/RIOT,basilfx/RIOT,kaspar030/RIOT,smlng/RIOT,kYc0o/RIOT,ant9000/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,yogo1212/RIOT,authmillenon/RIOT,ant9000/RIOT,jasonatran/RIOT,ant9000/RIOT,yogo1212/RIOT,toonst/RIOT,OlegHahm/RIOT,basilfx/RIOT,OTAkeys/RIOT,miri64/RIOT,smlng/RIOT,kYc0o/RIOT,jasonatran/RIOT,kaspar030/RIOT,kYc0o/RIOT,authmillenon/RIOT