mac80211_hwsim: Set VHT capabilities only for the 5.2 GHz band

Previously, VHT capabilities and supported MCSs where set for all
bands, although VHT is only allowed on 5.2 GHz band. Fix it.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>