mac80211: add VHT support for IBSS

Add VHT support for IBSS. Drivers could activate
this feature by setting NL80211_EXT_FEATURE_VHT_IBSS
flag.

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