[AVR32] extint: change set_irq_type() handling

Update the AVR32 EIC code to use the new __set_irq_handler_unlocked()
call, getting rid of one more instance of this widespread problem.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>