ath5k: Fix TX queues stopping

It does not make sense to stop queues for NF calibration. This will not stop
transmissions from the card, if there are queued packets.

If we run out of TX buffers we need to stop all queues, not only one.

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