ath9k: stop rx after tx

Completing frame transmission can fail if the rx engine is stopped
prematurely, as the hw might be waiting for an ACK from the other side.
Shutting down tx before rx might make the DMA shutdown more reliable.

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