wifi: iwlwifi: mld: validate aux sta before flush in stop_nan

wifi: iwlwifi: mld: validate aux sta before flush in stop_nan

iwl_mld_stop_nan() calls iwl_mld_flush_link_sta_txqs() without checking
that aux_sta.sta_id is valid. The DW end handler correctly guards this
with a WARN_ON check. Add the same defensive check to stop_nan for
consistency and to avoid sending a flush command with an invalid sta_id.

Signed-off-by: Daniel Gabay <daniel.gabay@intel.com>
Reviewed-by: Ilan Peer <ilan.peer@intel.com>
Link: https://patch.msgid.link/20260512222731.76b3db062a5e.I0567cfeb915c38c517eb6e1829d78c9fa8653c1f@changeid
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>