at86rf230: add irq type configuration option

Add option to at86rf230 platform data to configure the type of the
interrupt used by the driver.

Signed-off-by: Sascha Herrmann <sascha@ps.nvbi.de>
at86rf230: changed irq handling to not disable irq while handling it

Remove the calls to disable_irq_nosync() and enable_irq() in the irq
handler functions. 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. 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>