mac80211: minstrel_ht: pick only supported rates for sta and group max*rates

minstrel_ht initializes max_tp_rate max_tp_rate2 and max_prob_rate to
zero both for minstrel_ht_sta and minstrel_mcs_group_data.
This is wrong since there is no guarantee that the 1st rate of any
group is supported.

Signed-off-by: Karl Beldan <karl.beldan@rivierawaves.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>