nl80211: don't return err unconditionally in nl80211_start_ap()

When a memory leak was fixed, a return err was changed to goto err,
but, accidentally, the if (err) was removed, so now we always exit at
this point.

Fix it by adding if (err) back.

Fixes: 9951ebfc ("nl80211: fix potential leak in AP start")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200626124931.871ba5b31eee.I97340172d92164ee92f3c803fe20a8a6e97714e1@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>