at86rf230: add irq type configuration option

Add option to at86rf230 platform data to configure the type of the
interrupt used by the driver. The irq polarity of the device will
be configured accordingly.

Signed-off-by: Sascha Herrmann <sascha@ps.nvbi.de>
at86rf230: change irq handling to prevent lockups with edge type irq

Implemented separate irq handling for edge and level type interrupt
configuration. For edge type interrupts calls to disable_irq_nosync()
and enable_irq() are removed. The at86rf230 resets the irq line only
after the irq status register is read. Disabling the irq can lock the
driver in situations where a irq is set by the radio while the driver
is still reading the frame buffer.

With irq_type configuration set to 0 the original behavior is
preserverd.

Additional the irq filter register is set to filter out all unused
interrupts and the irq status register is read in the probe
function to clear the irq line.

Signed-off-by: Sascha Herrmann <sascha@ps.nvbi.de>