mac80211: ibss - remove not authorized station earlier

A station which is not authorized has to be purged earlier
to give it a chance to re-try to establish an IBSS/RSN
session soon. Set the timeout to 10 seconds.

Some refactoring has also been done to allow the IBSS
submodule to have its own expiring function.

Reported-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>