ath6kl: Add del_station cfg80211_ops

hostapd/wpa_supplicant AP mode uses this operation to flush the station
entries. Implement this in ath6kl to avoid unnecessary warnings from
NL80211_CMD_DEL_STATION failing.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>