mac80211: check IEEE80211_HW_QUEUE_CONTROL in ieee80211_check_queues()

Commit 3a25a8c8 (mac80211: add improved HW queue control) introduced a
bug when running in AP mode without the IEEE80211_HW_QUEUE_CONTROL
flag set.  The ieee80211_check_queues() function always returns
-EINVAL, preventing AP mode from starting.  To fix this, check whether
this flag is set before checking if cab_queue is set properly.

Signed-off-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>