wil6210: treat broadcast bssid as "disconnect all"

Hostapd request disconnect for broadcast bssid when it
wants to disconnect all stations from the AP.

Detect this and really disconnect all connected stations.

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>