ieee802154/adf7242: defer destroy_workqueue call

There is a possible race condition (use-after-free) like below

  (FREE)                     |  (USE)
  adf7242_remove             |  adf7242_channel
   cancel_delayed_work_sync  |
    destroy_workqueue (1)    |   adf7242_cmd_rx
                             |    mod_delayed_work (2)
                             |

The root cause for this race is that the upper layer (ieee802154) is
unaware of this detaching event and the function adf7242_channel can
be called without any checks.

To fix this, we can add a flag write at the beginning of adf7242_remove
and add flag check in adf7242_channel. Or we can just defer the
destructive operation like other commit 3e0588c291 ("hamradio: defer
ax25 kfree after unregister_netdev") which let the
ieee802154_unregister_hw() to handle the synchronization. This patch
takes the second option.

Fixes: 58e9683d14 ("net: ieee802154: adf7242: Fix OCL calibration
runs")
Signed-off-by: Lin Ma <linma@zju.edu.cn>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Link: https://lore.kernel.org/r/20220808034224.12642-1-linma@zju.edu.cn
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>

drivers/net/ieee802154/adf7242.c

@@ -1310,10 +1310,11 @@ static void adf7242_remove(struct spi_device *spi)
 
 	debugfs_remove_recursive(lp->debugfs_root);
 
+	ieee802154_unregister_hw(lp->hw);
+
 	cancel_delayed_work_sync(&lp->work);
 	destroy_workqueue(lp->wqueue);
 
-	ieee802154_unregister_hw(lp->hw);
 	mutex_destroy(&lp->bmux);
 	ieee802154_free_hw(lp->hw);
 }