mac80211: Helper function for marking STA authenticated

Authentication exchange can be completed in both TX and RX paths for
SAE, so move this common functionality into a helper function to avoid
having to implement practically the same operations in two places when
extending SAE implementation in the following commits.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>