mac80211: automatically free sta struct when insertion fails

When STA structure insertion fails, it has been allocated but isn't
really alive yet, it isn't reachable by any other code and also can't
yet have much configured. This patch changes the code so that when
the insertion fails, the resulting STA pointer is no longer valid
because it is freed.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>