at86rf230: add warning if edge-triggered irq

While testing I experience a deadlock while using the at86rf233 on a
raspberry pi. The reason was an edge triggered gpio irq because the irq
triggered while irq was disabled. This issue doesn't happend on a level
triggered irq because the irq will hit after calling enable_irq.

This patch adds a warning that it's not recommended to use a edge-triggered
irq type. Also change the examples to high-level irqtype.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>