ath9k: Fix flushing in MCC mode

When we are attempting to switch to a new
channel context, the TX queues are flushed, but
the mac80211 queues are not stopped and traffic
can still come down to the driver.

This patch fixes it by stopping the queues
assigned to the current context/vif before
trying to flush.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>