cfg80211/nl80211: Send AssocReq IEs to user space in AP mode

When user space SME/MLME (e.g., hostapd) is not used in AP mode, the
IEs from the (Re)Association Request frame that was processed in
firmware need to be made available for user space (e.g., RSN IE for
hostapd). Allow this to be done with cfg80211_new_sta().

Add a comment pointing out the use of enum station_info_flags for
all new struct station_info fields. In addition, memset the sinfo
buffer to zero before use on all paths in the current tree to avoid
leaving uninitialized pointers in the data.