mac80211: do not transmit frames on unconfigured 4-addr vlan interfaces

If frames are transmitted on 4-addr ap vlan interfaces with no station,
they end up being transmitted unencrypted, even if the ap interface
uses WPA. This patch add some sanity checking to make sure that this
does not happen.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>