rt2x00: check against flushing empty queue

We have check if queue is not empty when start flushing queues on
by mac80211 callback, but we also can start flushing queues by internal
driver calls. So move check into rt2x00queue_flush_queue() to assure
we do not flush empty queue anytime.

Additionally add warning if we start to kick empty queue as in such
situation we set wrong index in the HW queue, what can confuse the HW
and have various negative consequences.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>