mac80211/minstrel_ht: improve max_prob_rate selection

max_prob_rate should be selected to be very reliable, however limiting
it to single-stream on 3-stream devices is a bit much.
Allow max_prob_rate to use one stream less than the max_tp_rate.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>