mac80211/cfg80211: HT capabilities for NEW_STA

Allow userspace (e.g., hostapd) to set HT capabilities for associated
STAs. This is based on a patch from Zhu Yi <yi.zhu@intel.com> (only
the NL80211_ATTR_HT_CAPABILITY for NEW_STA part is included here).

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>