iwlwifi: mvm: IBSS: use BE FIFO for multicast

Back in commit 4d339989 ("iwlwifi: mvm: support ibss in dqa mode")
we changed queue selection for IBSS to be:

    if (ieee80211_is_probe_resp(fc) || ieee80211_is_auth(fc) ||
        ieee80211_is_deauth(fc))
            return IWL_MVM_DQA_AP_PROBE_RESP_QUEUE;
    if (info->hw_queue == info->control.vif->cab_queue)
            return info->hw_queue;
    return IWL_MVM_DQA_AP_PROBE_RESP_QUEUE;

Clearly, the thought at the time must've been that mac80211 will
select the hw_queue as the cab_queue, so that we'll return and use
that, where we store the multicast queue for IBSS. This, however,
isn't true because mac80211 doesn't implement powersave for IBSS
and thus selects the normal IBSS interface AC queue (best effort).

This therefore always used the probe response queue, which maps to
the BE FIFO.

In commit cfbc6c4c ("iwlwifi: mvm: support mac80211 TXQs model")
we rethought this code, and as a consequence now started mapping the
multicast traffic to the multicast hardware queue since we no longer
relied on mac80211 selecting the queue, doing it ourselves instead.
This queue is mapped to the MCAST FIFO. however, this isn't actually
enabled/controlled by the firmware in IBSS mode because we don't
implement powersave, and frames from this queue can never go out in
this case.

Therefore, we got queue hang reports such as
https://bugzilla.kernel.org/show_bug.cgi?id=201707



Fix this by mapping the multicast queue to the BE FIFO in IBSS so
that all the frames can go out.

Fixes: cfbc6c4c ("iwlwifi: mvm: support mac80211 TXQs model")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>