at86rf230: fix irq setup

Commit 8eba0eef ("at86rf230: remove irq_type in
request_irq") removed the trigger configuration when requesting an irq,
and instead relied on the interrupt trigger to be properly configured
already. This does not seem to be an assumption that can be safely made,
since boards disable all interrupt triggers on boot.

On these boards, force the irq to trigger on rising edge, which is also
the default for the chip.

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>