mac80211: max_tp_rate2 management of minstrel_ht

I noticed a possible issue in the max_tp_rate2 management of
minstrel_ht. In particular, if we look up just among max_tp_rate2 of
each group it will be possible that the selected rate will not be the
mcs with second maximum throughput. I wrote this simple patch.

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>