ath9k: Handle holding descriptor in TX completion properly

If the current holding descriptor is the last one in the TX queue,
*and* it has been marked as STALE, then move it to the free list
and bail out, as it has already been processed.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>