cpu/efm32/periph_gpio: fix NUMOF_IRQS off-by-one error
cpu/efm32/periph_gpio: fix NUMOF_IRQS off-by-one error

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