mac80211: Remove a redundant mutex unlock

The below-mentioned commit changed the code to unlock *inside*
the function, but previously the unlock was *outside*. It failed
to remove the outer unlock, however, leading to double unlock.

Fix this.

Fixes: 33483a6b

 ("mac80211: fix missing unlock on error in ieee80211_mark_sta_auth()")
Signed-off-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>
Link: https://lore.kernel.org/r/20200221104719.cce4741cf6eb.I671567b185c8a4c2409377e483fd149ce590f56d@changeid


[rewrite commit message to better explain what happened]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>