mac802154: Fixes kernel oops when unloading a radio driver

Destroying the workqueue before unregistering the net device caused a
kernel oops

Signed-off-by: Koen Zandberg <koen@bergzand.net>
Acked-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>