iwx is now logging a lot of stuff to the kernel when associated at VHT
rates. It's my fault; the replacement tx rate APIs print out a warning
when you're handing the legacy rate /HT rate APIs a VHT rate.

However all of the supported chipsets in iwx will be pushing the
rate control entirely into firmware. We don't need to do per-frame
TX rate control like in previous chips.

So for now, just put in an if (rate == VHT) into the path and
a TODO comment. A few lines later the function will just skip the
rate assignment stuff for data frames, thus saving us the logging.
The other paths (control traffic, multicast traffic) uses legacy
rates / HT rates by default and won't trigger a warning log.

Yes, iwx_tx_fill_cmd() really does deserve a rate control logic
rewrite, but I want to fix this particular issue first.

Locally tested:

• AX210, STA mode, HT and VHT associations