carl9170: remove dead branch in op_conf_tx callback

This patch removes the error branch for (queue > ar->hw->queues).
It is no longer needed anymore as the "queue" value is validated by
cfg80211's parse_txq_params() before the driver code gets called.

Some background:
In the old days (linux 2.6 and early 3.x), the parse_txq_params()
function did not verify the "queue" value. That's why these drivers
had to do it.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>