nl80211: Update drv->ssid on connect/associate event based on BSS data

nl80211: Update drv->ssid on connect/associate event based on BSS data

On a connect nl80211 event, wpa_supplicant uses
wpa_driver_nl80211_get_ssid() to fetch the current associated SSID to
compare to existing configurations. However,
wpa_driver_nl80211_get_ssid() uses drv->ssid, which is a cached value.
It is set when we explicitly initial a connect request using
wpa_supplicant. If the association was initiated outside of
wpa_supplicant, we need another way to populate drv->ssid. This commit
sets drv->ssid based on cfg80211 BSS information on connect/associate
nl80211 events.

Signed-off-by: Ningyuan Wang <nywang@google.com>