ath9k: Fix channel switch time duration

Since the NoA duration is the maximum time the GO interface
can be offline, it needs to include the time take to
switch channels in the HW.

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