ath9k: Handle abnormal NAV in AP mode

Beacon transmission would get stuck if the NAV is
an invalid value for some reason. Check and correct
the NAV value in the HW when this happens.

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