staging: rtl8723au: ConstructBeacon(): Use struct ieee80211_mgmt to build beacon

This gets rid of a bunch of hard coded offsets and reduces the
dependency of the ugly rtw_get_*_from_ie() functions.

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