nl80211: add VHT support for set_bitrate_mask

Add VHT MCS/NSS set support for nl80211_set_tx_bitrate_mask().
This should be used mainly for test purpose, to check
different MCS/NSS VHT combinations.

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