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

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