ath5k: Always free tx buffers before reset

 * Always free tx buffers before reset, since we also empty hw queues.
 If we don't and a queue gets stuck, we'll never decrease txq_len and sw
 will keep thinking the queue is still stuck even after reset.

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>