at86rf230: rework irq_pol setting

This patch rework the irq_pol register setting for rising and falling
interrupt settings only. The default behaviour should be rising flag.

Also use IRQ_TYPE_* defines instead of IRQF_* defines. There is no
functionality change but irq_get_trigger_type returns IRQ_TYPE_* defines.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>