ieee802154/adf7242: defer destroy_workqueue call

In the Linux kernel, the following vulnerability has been resolved:

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 3e0588c291d6 ("hamradio: defer
ax25 kfree after unregister_netdev") which let the
ieee802154_unregister_hw() to handle the synchronization. This patch
takes the second option.

runs")