broadcom: Add prune event handler

WLC_E_PRUNE event will generate assoc-failure ubus event. Status field included in assoc-failure event will contain assoctiation failure status code as described in IEEE Std 802.11-2020 "Table 9-50—Status codes".

Format of the event:

{ "wifi.sta": {"ifname":"wl0","event":"assoc-failure","data":{"macaddr":"00:0a:52:06:2e:b9", "status":12}} }