ath5k: do not call ieee80211_stop_queue for queues not managed by mac80211

Instead of using ieee80211_stop_queue, check the configured tx queue
limit before calling ieee80211_get_buffered_bc.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>