prima: Flush pmksa cache for SAP when SAP stop

In certain case, the active SAP interface will be deleted by
__wlan_hdd_del_virtual_intf by upper layer. In that case,
Driver should flush pmksa in SAP cache.

Change-Id: I9a152df7a4c27aef82664f039fbca18259d6b63a
CRs-Fixed: 2635424