staging: rtl8723au: ConstructProbeRsp(): Use struct ieee80211_mgmt

Use ieee80211_mgmt to construct the frame instead of relying on hard
coded offsets.

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