mac802154: tx: make worker information static

This patch moves the worker information struct out of skb control block.
Instead control block we declare it static inside of tx.c file. We can do
that, because the worker can't be used twice at the same time. It's
protected by stop and wake netdev queue.

This patch fix an issue that the "struct ieee802154_xmit_cb" doesn't fit
into the skb control block on some kernel configuartion reported by
kbuild test robot.

It was introduced by commit fe24371d
("mac802154: tx: remove kmalloc in xmit hotpath").

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