Tx status reporting on mt7603 has a number of issues:

- the hardware can alter the first rate index, but it is not reported to
  the driver
- probing is very imprecise, because it alters the per-client rate set,
  but only considers info->status.rates for rate selection of a single probe
  packet
- short/long GI selection has limitations, which are not accurately reported
  to mac80211
- if rates are changed while packets are in flight, tx status reports for
  the old rate set might be processed based on the new selection

This led to very suboptimal rate selection with minstrel_ht.

This patch completely reworks tx status reporting to get rid of these
limitations:

- Store the previous and current rate set in the driver + the TSF value
  at the time of the switch.
- Use the tx status TSF value to determine which rate set needs to be used
  as reference.
- Report only short or long GI rates for a single status event, not a mix.
- The hardware reports the last used rate index. Use it along with the
  retry count to figure out what rate was used for the first attempt.
- Use the same retry count value for all rate slots to make this calculation
  work.
- Derive the probe rate from the current rateset instead of the skb cb
- Do not wait for a status report for the probe frame before removing the
  probe rate from the rate table. Do it immediately after it was referenced
  in a tx status report.
- Use the first half of the first rate retry budget for the probe rate
  in order to avoid using too many retries on that rate

With this patch, throughput under bad link conditions is improved
significantly, and there is a lot less rate fluctuation going on.
Signed-off-by: Felix Fietkau <nbd@nbd.name>