mac80211: fix setting IEEE80211_KEY_FLAG_RX_MGMT for AP mode keys

key->sta is only valid after ieee80211_key_link, which is called later
in this function. Because of that, the IEEE80211_KEY_FLAG_RX_MGMT is
never set when management frame protection is enabled.

Fixes: e548c49e ("mac80211: add key flag for management keys")
Cc: stable@vger.kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>