at86rf230: add force slotted operation bit

This patch adds a force setting of slotted operation bit. The atben
chips sometimes set these bit. The reason is unknown. Nevertheless we
don't support slotted operation so we set this bit now force while
probing.

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