staging: rtl8723au: OnBeacon23a() tidy up code

Clean up the excessive if() levels at the end, and use struct
ieee80211_mgmt to calculate pointers passed on to check_assoc_AP23a()

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