p54: remove dead branch in op_conf_tx callback

This patch removes the error branch for (queue > dev->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.

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