wifi: iwlwifi: mld: purge async notifications upon nic error

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

wifi: iwlwifi: mld: purge async notifications upon nic error

This fixes a kernel panic in reconfig failure:

1. we have a BSS connection
2. we have a NAN connection
3. FW error occurs
4. reconfig restores the BSS connection
5. however, restoring the NAN connection fails due to a FW error.
6. erroneously, ieee80211_handle_reconfig_failure is called and marks all
interfaces as not-in-driver (will be fixed in a different patch).
7. mac80211 frees the links of the BSS connection but doesn't tell the
driver about that, as it thinks that this vif is not in the driver.
8. in ieee80211_stop_device, *ALL* wiphy works are getting flushed
(erroneously?)
9. Therefore, async_handlers_wk is being executed, processing the
statistics notification that was received after we restored the BSS
connection.
10. the notification handler dereferences fw_id_to_bss_conf[id], which is
now a dangling pointer, as mac80211 already freed this link in (7).
11. On the first access to one of the links fields, we panic.

While this can and should be fixed by removing the call to
ieee80211_handle_reconfig_failure in (6), it is also not a good idea to
carry and maybe handle notifications from a dead FW.

We do purge the notifications when we stop the FW, but in reconfig
failure we stop the FW too late, after the notifications are processed.
In addition, async_handlers_wk can always be scheduled before the
reconfig work.

Purge the notifications immediately when transport notifies about a nic
error.