ath5k: Keep last descriptor in queue

If we return a TX descriptor to the pool of available descriptors, while a
queues TXDP still points to it we could potentially run into all sorts of
troube.

It has been suggested that there is hardware which can set the descriptors
done bit before it reads ds_link and moves on to the next descriptor. While the
documentation says this is not true for newer chipsets (the descriptor contents
are copied to some internal memory), we don't know about older hardware.

To be safe, we always keep the last descriptor in the queue, and avoid dangling
TXDP pointers. Unfortunately this does not fully resolve the problem - queues
still get stuck!

This is similar to what ath9k does.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>