mac80211: clear ifmgd->bssid only after building DELBA

ieee80211_set_disassoc() clears ifmgd->bssid before
building DELBA frames, resulting in frames with invalid
bssid ("00:00:00:00:00:00").

Fix it by clearing ifmgd->bssid only after building
all the needed frames.

After this change, we no longer need to save the
bssid (before clearing it), so remove the local array.

Reported-by: Ido Yariv <ido@wizery.com>
Cc: stable@vger.kernel.org
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>