mac80211: mlme: save ssid info to ieee80211_bss_conf while assoc

The ssid info of ieee80211_bss_conf is filled in ieee80211_start_ap()
for AP mode. For STATION mode, it is empty, save the info from struct
ieee80211_mgd_assoc_data, the struct ieee80211_mgd_assoc_data will be
freed after assoc, so the ssid info of ieee80211_mgd_assoc_data can not
access after assoc, save ssid info to ieee80211_bss_conf, then ssid info
can be still access after assoc.

Signed-off-by: Wen Gong <wgong@codeaurora.org>
Link: https://lore.kernel.org/r/1607312195-3583-2-git-send-email-wgong@codeaurora.org
[reset on disassoc]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>