staging: rtl8723au: OnBeacon23a(): Use struct ieee80211_mgmt

We always receive a struct ieee80211_mgmt frame here, using just
ieee80211_hdr doesn't make sense.

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