mac80211: in ADHOC don't update last_rx if sta is not authorized

It does not make sense to keep a station alive if it is not authorised
at all. If IBSS/RSN is used it could also be the case that something
went wrong during the keys exchange and the stations ended up in a not
recoverable state.

By not updating last_rx we are giving the station a chance to be
deleted and to start the key exchange once again from scratch.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>