staging: rtl8723au: OnAuth23a(): ieee80211.h-ify

Switch to using struct ieee80211_mgmt to obtain offsets. Again a
bizarre +4 offset was applied for the IE scan which doesn't make
sense, since this offset wasn't applied for the auth struct elements.

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