ath9k: Check for active GO in mgd_prepare_tx()

If a GO interface is active when we receive a
mgd_prepare_tx() call, then we need to send
out a new NoA before switching to a new context.

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