mac80211: do not drop tx nulldata packets on encrypted links

ieee80211_tx_h_select_key drops any non-mgmt packets without a key when
encryption is used. This is wrong for nulldata packets that can't be
encrypted and are sent out for probing clients and indicating 4-address
mode.

Reported-by: Sebastian Gottschall <s.gottschall@dd-wrt.com>
Fixes: a0761a30

 ("mac80211: drop data frames without key on encrypted links")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20201218191525.1168-1-nbd@nbd.name
Signed-off-by: Johannes Berg <johannes.berg@intel.com>