at86rf230: add irqmask mode setting

Since we support at86rf233 we need to ensure that basic operation
default values are the same. This patch always sets IRQ_MASK_MODE to 0
which is after reset 1 at the at86rf233 and 0 at the at86rf231.

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