cpu/sam0_common: adc: default to 0 for ADC_NEG_INPUT
cpu/sam0_common: adc: default to 0 for ADC_NEG_INPUT

kaspar030/RIOT,ant9000/RIOT,miri64/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,ant9000/RIOT,ant9000/RIOT,miri64/RIOT,jasonatran/RIOT,ant9000/RIOT,miri64/RIOT,jasonatran/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,jasonatran/RIOT,miri64/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,miri64/RIOT