mac80211: Track auth frame registrations on IBSS ifaces

Track userspace registrations for authentication
frames received on an IBSS interface. This field
will be used to decide whether or not to send
"open system" authentication frames when a new
station joins an adhoc network.

Signed-off-by: Will Hawkins <hawkinsw@opentechinstitute.org>
[redesign the code flow a bit]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>