mac80211: provide SSID in IBSS mode

Some drivers need SSID in AP and IBSS mode. AP SSID is provided
through BSS_CHANGED_SSID notification. There was no easy way to
do the same for IBSS. In IBSS mode SSID is known but was not
stored in BSS configuration. Extend the AP-mode functionality
to also work in IBSS mode.

Signed-off-by: Marek Puzyniak <marek.puzyniak@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>