Revert "rt2800: use TXOP_BACKOFF for probe frames"

commit 52a192362932f333a7ebafd581c4d9b81da2fec8 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>