mac80211: fix rate control tx handler for VHT rates

Handle VHT rates like HT ones, otherwise we easily trigger the pre-HT
rates WARN_ON(rc_rate->idx >= sband->n_bitrates) which will set
rc_rate->idx to -1.

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