at86rf230: cleanup and squash stack variable
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 <d03dbcdedb9639e397df9b8578e8fc8274f4e6c4@gmail.com>
Signed-off-by: Marcel Holtmann <44592b4eea36663c86b994bb0ea99d15309c1c7d@holtmann.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs