Shut down RX before TX - in theory, this should make the chip less likely
to get upset.

The Qualcomm Atheros reference design code goes through significant
hacks to shut down RX before TX.  It doesn't even try do do it in the
driver - it actually makes the DMA stop routines in the HAL shut down
RX before shutting down TX.

So, to make this work for chips that aren't the AR9380 and later, do
it in the driver.  Shuffle the TX stop/drain HAL calls to be called
*after* the RX stop HAL call.

Tested:

* AR5413 (STA)
* AR5212 (STA)
* AR5416 (STA)
* AR9380 (STA)
* AR9331 (AP)
* AR9341 (AP)

TODO:

* test ar92xx series NIC and the AR5210/AR5211, in case there's something
  even odder about those.