mac80211: fix single queue drivers

My queue management rework broke drivers that don't
have multiple AC queues and register a single queue
only, causing a warning:

WARNING: at net/mac80211/iface.c:162 ieee80211_check_queues

This was due to filling the queues wrongly and then
noticing the error when checking later.

Reported-and-Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>