[AVR32] extint: Set initial irq type to low level

David Brownell pointed out a mismatch in the avr32 extint code:

> I noticed a small glitch that's not fixed by this patch:  the
> initial type is falling edge, but IRQ_TYPE_NONE is mapped to
> IRQ_TYPE_LEVEL_LOW.  Potentially surprising.

Fix it by setting the initial type (and handler) to low level,
matching the meaning of IRQ_TYPE_NONE.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>