mac80211: add a 2-way Authentication challenge to IBSS mode

In IBSS mode, whenever a new station is added a 2-way authentication challenge
is performed. Actually this event can be used to recognise a new station joining
the cell even if its sta_info entry is already in the list.

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