mac80211: add specific-queue flushing support

If the HW supports IEEE80211_HW_QUEUE_CONTROL, allow
flushing only specific queues rather than all of them.

Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>