boards/mega-xplained: fix ADC line defintions
boards/mega-xplained: fix ADC line defintions

For the ATmega platform the ADC lines are not defined according to the GPIOs. Instead, they are defined from 0 to ADC_NUMOF-1, which in turn represents the bit in the corresponding ADC registers.  The mapping from the line number to the corresponding ADC register/bit combination is done implicitly by the periph/adc implementation.
authmillenon/RIOT,kYc0o/RIOT,kYc0o/RIOT,basilfx/RIOT,yogo1212/RIOT,x3ro/RIOT,authmillenon/RIOT,x3ro/RIOT,miri64/RIOT,jasonatran/RIOT,smlng/RIOT,kYc0o/RIOT,kYc0o/RIOT,OlegHahm/RIOT,ant9000/RIOT,OlegHahm/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kYc0o/RIOT,miri64/RIOT,jasonatran/RIOT,kaspar030/RIOT,smlng/RIOT,ant9000/RIOT,smlng/RIOT,yogo1212/RIOT,smlng/RIOT,jasonatran/RIOT,ant9000/RIOT,toonst/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,ant9000/RIOT,x3ro/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,authmillenon/RIOT,basilfx/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,miri64/RIOT,basilfx/RIOT,yogo1212/RIOT,yogo1212/RIOT,OlegHahm/RIOT,toonst/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,toonst/RIOT,toonst/RIOT,basilfx/RIOT,x3ro/RIOT,x3ro/RIOT,kaspar030/RIOT,authmillenon/RIOT,ant9000/RIOT,smlng/RIOT,kaspar030/RIOT,miri64/RIOT,authmillenon/RIOT,miri64/RIOT,yogo1212/RIOT,OTAkeys/RIOT,kaspar030/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,toonst/RIOT,basilfx/RIOT,jasonatran/RIOT,RIOT-OS/RIOT