cpu/atmega_common/periph: Fixed bug in PCINT
cpu/atmega_common/periph: Fixed bug in PCINT

The bank index and the pin number are not necessarily identical. For all
PCINT banks except for bank 3 bank_idx was used therefore. It was likely
just forgotten to update that for bank 3 as well.

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