cfg80211: in bitrate_mask, rename mcs to ht_mcs

Rename NL80211_TXRATE_MCS to NL80211_TXRATE_HT and also
rename mcs to ht_mcs in struct cfg80211_bitrate_mask.

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
[reword commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>