rt2800: change default retry settings

We do not have option to set per frame retry count. We have only global
TX_RTY_CFG registers which specify the number or retries. Set setting
of that register to value that correspond rate control algorithm number
of frame post (number of retries + 1), which is 3 for aggregated frames.
This should help with big amount of retries on bad conditions, hence
mitigate buffer-bloat like problems.

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