mac80211: send auth in IBSS only if userspace isn't handling it

Check the auth frame registration count before sending
"open system" authentication messages when a new station
registers on a particular IBSS network. This stops us
from sending out multiple authentication messages with
different authentication algorithms.

Signed-off-by: Will Hawkins <hawkinsw@opentechinstitute.org>
[reword commit message a bit]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>