wifi: mac80211: mlme: Fix missing unlock on beacon RX

wifi: mac80211: mlme: Fix missing unlock on beacon RX

Commit 5fe1941690ca ("wifi: mac80211: mlme: use correct link_sta")
switched to link station instead of deflink and added some checks to do
that, which are done with the 'sta_mtx' mutex held. However, the error
path of these checks does not unlock 'sta_mtx' before returning.

Fixes: 5fe1941690ca ("wifi: mac80211: mlme: use correct link_sta")
Signed-off-by: Rafael Mendonca <rafaelmendsr@gmail.com>
Link: https://lore.kernel.org/r/20220924184042.778676-1-rafaelmendsr@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>