wifi: mac80211: netdev compatible TX stop for iTXQ drivers

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>