mac80211: Allow userspace to register for auth frames in IBSS

Set the necessary flags to allow user space applications
to register for authentication frames on IBSS interfaces.
This is useful for situations where userspace applications
want to control key negotiation between stations.

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