ath9k: Implement op_flush()

When op_flush() is called with no drop (drop=false), the driver
tries to tx as many frames as possible in about 100ms on every
hw queue. During this time period frames from sw queue are also
scheduled on to respective hw queue.

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>