mac80211: do not override fixed_rate_idx in minstrel_ht_update_stats

Do not override max_tp_rate, max_tp_rate2 and max_prob_rate configured
according to fixed_rate in minstrel_ht_update_stats throughput computation

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>