avr32: Cleanup eic_set_irq_type()

No need to fiddle in irq_desc. The trigger mask can be written back
into irq_data. Return IRQ_SET_MASK_OK_NOCOPY, so the generic code wont
overwrite it again.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>