wifi: mac80211: clean up band switch in duration

Most devices now do duration calculations, so we don't hit
this code at all any more. Clearly the approach of warning
at compile time here when new bands are added didn't work,
the new bands were just added with "TODO". Clean it up, it
won't matter for new bands since they'll just not have any
need to calculate durations in software.

While at it, also clean up and unify the code a bit.

Link: https://msgid.link/20240129194108.70a97bd69265.Icdd8b0ac60a382244466510090eb0f5868151f39@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/tx.c

@@ -133,6 +133,7 @@ static __le16 ieee80211_duration(struct ieee80211_tx_data *tx,
 	mrate = sband->bitrates[0].bitrate;
 	for (i = 0; i < sband->n_bitrates; i++) {
 		struct ieee80211_rate *r = &sband->bitrates[i];
+		u32 flag;
 
 		if (r->bitrate > txrate->bitrate)
 			break;
@@ -145,28 +146,24 @@ static __le16 ieee80211_duration(struct ieee80211_tx_data *tx,
 
 		switch (sband->band) {
 		case NL80211_BAND_2GHZ:
-		case NL80211_BAND_LC: {
-			u32 flag;
+		case NL80211_BAND_LC:
 			if (tx->sdata->deflink.operating_11g_mode)
 				flag = IEEE80211_RATE_MANDATORY_G;
 			else
 				flag = IEEE80211_RATE_MANDATORY_B;
-			if (r->flags & flag)
-				mrate = r->bitrate;
 			break;
-		}
 		case NL80211_BAND_5GHZ:
 		case NL80211_BAND_6GHZ:
-			if (r->flags & IEEE80211_RATE_MANDATORY_A)
-				mrate = r->bitrate;
+			flag = IEEE80211_RATE_MANDATORY_A;
 			break;
-		case NL80211_BAND_S1GHZ:
-		case NL80211_BAND_60GHZ:
-			/* TODO, for now fall through */
-		case NUM_NL80211_BANDS:
+		default:
+			flag = 0;
 			WARN_ON(1);
 			break;
 		}
+
+		if (r->flags & flag)
+			mrate = r->bitrate;
 	}
 	if (rate == -1) {
 		/* No matching basic rate found; use highest suitable mandatory