wifi: iwlwifi: mld: refactor purging async notifications

wifi: iwlwifi: mld: refactor purging async notifications

To cancel all async notifications, we need to:
- cancel async_handlers_wk
- empty async_handlers_list

Instead of having the callers to do both, do it in
iwl_mld_purge_async_handlers_list and rename it accordingly.

Note that the caller iwl_cleanup_mld didn't cancel the work, but it is
harmless.

Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Link: https://patch.msgid.link/20250423091408.e98aed77e836.Id9f858d9d553d406a24165b09db830df111befce@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>