ath9k: Fix HW scan abort

Instead of using ATH_CHANCTX_EVENT_ASSIGN to abort
a HW scan when a new interface becomes active, use the
mgd_prepare_tx() callback. This allows us to make
sure that the GO's channel becomes operational by
using flush_work().

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