iwlwifi: mvm: add multicast station

iwlwifi: mvm: add multicast station

Currently multicast queue is associated with the broadcast
station.

This raises quite a few issues:

The multicast queue has a special treatment:
- It is sent in the MAC context command
- It is excluded from tfd_queue_mask

In DQA mode we end up enabling two queues - the probe response
queue and the multicast queue - with the same station (broadcast)
and TID while in DQA mode it should be unique RA-TID.
Firmware will enforce it for a000 devices, so this allocation
will fail.

In addition, in a000 devices the FW will set the FIFO and not
the driver. So there is a need for FW to know when we enable
the queue that it is multicast queue so it will be bound to
the multicast FIFO. There is no such way in current design.

In order to simplify driver and firmware handling of this queue
create a multicast station.
This solves the unique RA-TID issue in the short term and serves
as preparation for the long term.

In the long term we will also add a flag marking this station for
the FW as the multicast station.
Once we will do that the FW will know this is the multicast queue
immediately when it is added and bind it to the correct FIFO.
It will also enable removing the special treatment of the
queue in the MAC context command.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>