iwlwifi: mvm: support TX on MONITOR iface

When trying to TX through a monitor interface, the
conditions in iwl_mvm_tx_skb_non_sta() don't match
and the frame tries to go out from an usued TXQ.

Add a check for monitor iface, and use the AUX queue
in such a case.

In non-DQA mode the frame is sent through the
static-allocated queues anyway, so the problem is
in DQA mode only.

Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>