mac80211: in IBSS use the Auth frame to trigger STA reinsertion

In case of a node re-joining the cell the sta_info structure belonging to it is
first destroyed and then reinserted. In this way its internal state is reset.

The joining operation is recognised thank the Auth frame being received.

This operation is helpful in case of a node being rebooted that is joining the
ad-hoc cell again, before its purge timeout on other nodes expires.

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>