ath9k: fix tx queue scheduling after channel changes

commit ec30326ea773900da210c495e14cfeb532550ba2 upstream.

Otherwise, if queues are full during a scan, tx scheduling does not
resume after switching back to the home channel.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>