at86rf230: cleanup and squash stack variable

I had this variable because I thought it would be protected by
disable/enable irq but this is not true. It's protected by stop/wake
netdev queue which is called by ieee802154_xmit_complete.

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