ath9k: stop rx dma before stopping tx

It has been found that active Rx can interfere with stopping tx DMA, which
could result in at least parts of those "Failed to stop Tx DMA!" messages.
Stopping rx before tx should prevent that.

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