rt2800: don't set ht parameters for non-aggregated frames

Do not set ampdu_density and ba_size for frames without AMPDU bit i.e.
frames that will not be aggregated to AMPDU.

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