wifi: mac80211: use ap_addr for 4-address NULL frame destination

wifi: mac80211: use ap_addr for 4-address NULL frame destination

Currently ieee80211_send_4addr_nullfunc() uses deflink.u.mgd.bssid
for addr1 and addr3 fields. In MLO configurations, deflink.u.mgd.bssid
represents link 0's BSSID and is not updated when link 0 is not an
assoc link. This causes 4-address NULL frames to be sent to the
wrong address, preventing WDS AP_VLAN interface creation on the peer AP.

To fix this use sdata->vif.cfg.ap_addr instead, which contains the AP's MLD
address populated during authentication/association and remains
valid regardless of which links are active.

This ensures 4-address NULL frames reach the correct AP, allowing
proper WDS operation over MLO connections.

Co-developed-by: Sathishkumar Muruganandam <quic_murugana@quicinc.com>
Signed-off-by: Sathishkumar Muruganandam <quic_murugana@quicinc.com>
Signed-off-by: Tamizh Chelvam Raja <tamizh.raja@oss.qualcomm.com>
Link: https://patch.msgid.link/20260326164723.553927-3-tamizh.raja@oss.qualcomm.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>