drivers: wifi: nxp: fix sap stations list shows empty in open mode

Start ap with open mode, external station connected to sap,
wifi ap stations list shows empty, because no
'WLAN_REASON_UAP_CLIENT_CONN' is received, skip add
station infomation to station list,so station list shows empty.
Fix issue by trigger this event with
'WLAN_REASON_UAP_CLIENT_ASSOC'in open mode.

Signed-off-by: Rex Chen <rex.chen_1@nxp.com>