ath9k: Fix drain txq failure in flush

While draining the txq in flush, the buffers can be
added into the tx queue by tx_tasklet which leads to
unneccesary chip reset.

This issue was originially found with AR9382 and
running heavy uplink udp traffic with higher bandwidth
and doing frequent bgscan.

Cc: stable@kernel.org
Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>