iwlwifi: mvm: Send non offchannel traffic via AP sta

TDLS discovery response frame is a unicast direct frame to the peer.
Since we don't have a STA for this peer, this frame goes through
iwl_tx_skb_non_sta(). As the result aux_sta and some completely
arbitrary queue would be selected for this frame, resulting in a queue
hang.  Fix that by sending such frames through AP sta instead.

Signed-off-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>