In some cases we may want to stop the queues of a single vif (for
instance during a channel-switch).  Add a function that stops all the
queues that are assigned to a vif.  If a queue is assigned to more
than one vif, the corresponding netdev subqueue of the other vif(s)
will also be stopped.  If the HW doesn't set the
IEEE80211_HW_QUEUE_CONTROL flag, then all queues are stopped.

Also add a corresponding function to wake the queues of a vif back.
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>