mac80211_hwsim: Advertise support for VHT IBSS

VHT can be used with IBSS without needing any additional changes in
mac80211_hwsim, so start claiming support for this to increase test
coverage.

Signed-off-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>