wil6210: fix disconnect handling for AP

For the AP-like interface, if "disconnect all" requested,
every station should be deleted with cfg80211_del_sta().
Implement this behavior.

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>