staging: rtl8723au: issue_beacon23a(): Use struct ieee80211_mgmt to build beacon

Why on Earth we have two functions in the driver constructing beacon
frames is beyond me ... but one step at a time

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