Revert "rt2800: use TXOP_BACKOFF for probe frames"

This reverts commit fb47ada8.

In some situations when we set TXOP_BACKOFF, the probe frame is
not sent at all. What it worse then sending probe frame as part
of AMPDU and can degrade 11n performance to 11g rates.

Cc: stable@vger.kernel.org
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>