rt2800: status based rate flags for nomatch case

We use skb_desc->tx_rate_flags from entry as rate[].flags even if
skb does not match status. Patch corrects flags and also fixes
mcs for legacy rates.

rt2800_rate_from_status() is based on Felix's mt76
mt76x2_mac_process_tx_rate() function.

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