Fix EXTERNAL_NUM_INTERRUPTS for atmega128rfa1 and atmega256rfr2

This makes all 8 interrupts on these chips work. This change was originally submitted by @penguin359 in (the now outdated) #1399.

@cmaglie, I'd be grateful if you could quickly merge this trivial fix, since it currently blocks our users from using all interrupts with our Pinoccio board.