mac80211: minstrel_ht: fix per-group max throughput rate initialization

The group number needs to be multiplied by the number of rates per group
to get the full rate index

Fixes: 5935839a ("mac80211: improve minstrel_ht rate sorting by throughput & probability")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20190820095449.45255-1-nbd@nbd.name


Signed-off-by: Johannes Berg <johannes.berg@intel.com>