at86rf230: detailed edge triggered irq warning

This patch introduce a more detailed information why edge triggered
irq's are currently not recommended. It could be that rising/falling
edge detection can happen while the irq is disabled.

Suggested-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>