ath9k: fix DMA stop sequence for AR9003+

commit 300f77c08ded96d33f492aaa02549103852f0c12 upstream.

AR93xx and newer needs to stop rx before tx to avoid getting the DMA
engine or MAC into a stuck state.
This should reduce/fix the occurence of "Failed to stop Tx DMA" logspam.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>