mac80211: insert stations before adding to driver

There's a race condition in mac80211 because we add stations
to the internal lists after adding them to the driver, which
means that (for example) the following can happen:
 1. a station connects and is added
 2. first, it is added to the driver
 3. then, it is added to the mac80211 lists

If the station goes to sleep between steps 2 and 3, and the
firmware/hardware records it as being asleep, mac80211 will
never instruct the driver to wake it up again as it never
realized it went to sleep since the RX path discarded the
frame as a "spurious class 3 frame", no station entry was
present yet.

Fix this by adding the station in software first, and only
then adding it to the driver. That way, any state that the
driver changes will be reflected properly in mac80211's
station state. The problematic part is the roll-back if the
driver fails to add the station, in that case a bit more is
needed. To not make that overly complex prevent starting BA
sessions in the meantime.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>