iwx: don't log VHT rate lookups in iwx_tx_fill_cmd() for data frames

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

Differential Revision:	https://reviews.freebsd.org/D52766
Reviewed by:	thj

sys/dev/iwx/if_iwx.c

@@ -5501,6 +5501,9 @@ iwx_tx_fill_cmd(struct iwx_softc *sc, struct iwx_node *in,
 		/* for non-data, use the lowest supported rate */
 		ridx = min_ridx;
 		*flags |= IWX_TX_FLAGS_CMD_RATE;
+	} else if (ni->ni_flags & IEEE80211_NODE_VHT) {
+		/* TODO: VHT - the ridx / rate array doesn't have VHT rates yet */
+		ridx = iwx_min_basic_rate(ic);
 	} else if (ni->ni_flags & IEEE80211_NODE_HT) {
 		ridx = iwx_mcs2ridx[ieee80211_node_get_txrate_dot11rate(ni)
 		    & ~IEEE80211_RATE_MCS];