mac80211: fix missing unlock on error in ieee80211_mark_sta_auth()

Add the missing unlock before return from function
ieee80211_mark_sta_auth() in the error handling case.

Cc: stable@vger.kernel.org
Fixes: fc107a93

 ("mac80211: Helper function for marking STA authenticated")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
[use result variable/label instead of duplicating]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>