cpu/qn908x: Fix triggering GPIO IRQ.
cpu/qn908x: Fix triggering GPIO IRQ.

The IRQ for each GPIO port needs to be enabled in the NVIC on top of
enabling the corresponding bit in the GPIO port.

This was not caught in tests before because I was testing with a larger
stack of commits (including UART and timers) which also had this fix.

Manually poking the GPIOs while using tests/periph_gpio now properly
fires the interrupts.

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