mac80211: skip legacy rate mask handling for VHT rates

The rate mask code currently assumes that a rate is legacy if
IEEE80211_TX_RC_MCS is not set. This might be the cause of bogus VHT
rates being reported with minstrel_ht.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>