ath9k: Send oneshot NoA

This patch makes sure that a GO interface
sends out a new NoA schedule with 200ms duration
when mgd_prepare_tx() is called.

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