staging: rtl8723au: collect_bss_info23a(): Use struct ieee80211_mgmt

Somplify code by using struct ieee80211_mgmt to calculate offsets

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