mac80211: minstrel_ht: fix MCS_GROUP_RATES usage

Commit 5935839a ("mac80211: improve minstrel_ht rate sorting by
throughput & probability") replaced the constant 8 with MCS_GROUP_RATES
when getting the number of streams of an HT MCS. See commit 7a5e3fa2
("mac80211: minstrel_ht: replace some occurences of MCS_GROUP_RATES").

Fixes: 5935839a ("mac80211: improve minstrel_ht rate sorting by throughput & probability")
Signed-off-by: Karl Beldan <karl.beldan@rivierawaves.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>