mac80211: reply to AUTH with DEAUTH if sta allocation fails in IBSS

Whenever a host gets an AUTH frame it first allocates a new
station and then replies with another AUTH frame. However,
if sta allocations fails the host should send a DEAUTH frame
instead to tell the other end that something went wrong.

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
[reword commit message a bit]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>