[AVR32] ext int fixes

Bugfixes for external irq handler set_irq_type():

 - If set_irq_type() can't set the type, don't change anything!

 - It's not OK to change the flow handler as part of set_irq_type(),
   among other issues that violates spinlock rules.  Instead, we can
   call the relevant handler when we demux the external interrupts.

 - The external irq demux has no need to grab the spinlock.  And in
   fact grabbing it that way was wrong, since that code might be
   pre-empted by an irq at a different priority level, and that code
   might then have tried to grab that spinlock...

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