mac80211: use capped prob when computing throughputs

Commit 3e8b1eb2 "mac80211/minstrel_ht: improve rate selection stability"
introduced a local capped prob in minstrel_ht_calc_tp but omitted to use
it to compute the per rate throughput.

Signed-off-by: Karl Beldan <karl.beldan@rivierawaves.com>
Cc: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>