iwlwifi: mvm: synchronize TID queue removal

When we mark a TID as no longer having a queue, there's no
guarantee the TX path isn't using this txq_id right now,
having accessed it just before we reset the value. To fix
this, add synchronize_net() when we change the TIDs from
having a queue to not having one, so that we can then be
sure that the TX path is no longer accessing that queue.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>