iwlwifi: mvm: Set Tx rate and flags when there is not station

When a frame is transmitted without a station, need to set the rate
and flags in the Tx command, as the FW does not have any information as
to what rate and flags should be used for this frame.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>