commit 300f77c0 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>
[bwh: Backported to 3.2:
 - Also move initialisation of ret to match upstream
 - ath_drain_all_txq() takes a second parameter]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>