mac80211: fix crash with single-queue drivers

Larry (and some others I think) reported that with
single-queue drivers mac80211 crashes when waking
the queues. This happens because we allocate just
a single queue for each virtual interface in case
the driver doesn't have at least 4 queues, but the
code stopping/waking the virtual interface queues
wasn't taking this into account.

Reported-by: Larry Finger <Larry.Finger@lwfinger.net>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>