Properly handle TX stop for internal queues (iTXQs) within mac80211.

mac80211 must not stop netdev queues when using mac80211 iTXQs.
For these drivers the netdev interface is created with IFF_NO_QUEUE.

While netdev still drops frames for IFF_NO_QUEUE interfaces when we stop
the netdev queues, it also prints a warning when this happens:
Assuming the mac80211 interface is called wlan0 we would get
"Virtual device wlan0 asks to queue packet!" when netdev has to drop a
frame.

This patch is keeping the harmless netdev queue starts for iTXQ drivers.
Signed-off-by: Alexander Wetzel <alexander@wetzel-home.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>