staging: rtl8723au: OnAssocReq23a(): Use cfg80211_find_ie() to locate HT caps

Use cfg80211 interface to search for HT capabilities.

This was the last user of struct rtw_ieee802_11_elems, which can now
be removed.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>