iwlwifi: mvm: Do not consider invalid HW queues in queue mask

The iwl_mvm_mac_get_queues_mask() added vif->hw_queue[ac] to the
queue mask although it might be set to IEEE80211_INVAL_HW_QUEUE.
Fix it.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>