mac80211: pass station to ieee80211_vht_cap_ie_to_sta_vht_cap

Like with HT, make things a bit simpler in future patches by
passing the station to ieee80211_vht_cap_ie_to_sta_vht_cap()
instead of the vht_cap pointer. Also disable VHT here if HT
isn't supported.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>