cpu/saml21: cannot use several EXTI

[dylad]

I was playing with EXTI on SAML21 and I notice an annoying problem.
Hardware : saml21-xpro
Software : tests/periph_gpio

First I try to register a EXTI on any GPIO (PA18 for example -> EXTI 2)
Registration is ok, I attach a button to it and it works fine.
Now I try to register a second pin (PA17 -> EXTI 1)
registration is ok, I attach a second button but no IRQ trig. (First button still working).

Now I reset the board, I initialize PA17 first as EXTI, the button works fine. Then I initiliaze PA18 as EXTI and this time PA18 doesn't work.

It seems the call to isr_eic() occurred only for the first register EXTI.

[dylad]

#6932 fixes this issue so I'm closing it.