mac80211: use offchannel queue only when supported

Commit 3a25a8c8 ("mac80211: add improved HW queue control")
added support for offchannel queue mapping. However, this
mapping is only valid when the driver supports
IEEE80211_HW_QUEUE_CONTROL.

Check whether the driver supports IEEE80211_HW_QUEUE_CONTROL
before setting the hw_queue to the mapped offchannel queue.

(This patch doesn't have any actual effect, because
hw_queue is overridden in ieee80211_tx() anyway,
so this is merely some cleanup)

Signed-off-by: Eliad Peller <eliad@wizery.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>