mac80211: minstrel_ht: add support for OFDM rates on non-HT clients

The legacy minstrel code is essentially unmaintained and receives only very
little testing. In order to bring the significant algorithm improvements from
minstrel_ht to legacy clients, this patch adds support for OFDM rates to
minstrel_ht and removes the fallback to the legacy codepath.
This also makes it work much better on hardware with rate selection constraints,
e.g. mt76.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20210115120242.89616-3-nbd@nbd.name
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rc80211_minstrel.h

@@ -152,6 +152,7 @@ struct minstrel_priv {
 	unsigned int lookaround_rate_mrr;
 
 	u8 cck_rates[4];
+	u8 ofdm_rates[NUM_NL80211_BANDS][8];
 
 #ifdef CONFIG_MAC80211_DEBUGFS
 	/*