at86rf230: remove unnecessary / dead code

In at86rf230_probe() lp was first set to dev->priv and a few lines later
dev->priv was set to lp again, without changing lp in between. The call
to ieee802154_unregister_device() before err_irq: was unreachable.

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

Hard code the interrupt type of the at86rf230 to rising edge type and
remove the calls to disable_irq_nosync() and enable_irq() from the
isr and the irqwork function. 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>