staging: rtl8723au: Make rtw_check_bcn_info23() take a ieee80211_mgmt pointer

Clean up the code by passing a struct ieee80211_mgmt pointer instead
of a raw frame pointer. In addition check we receive a beacon frame,
anything else here is an error.

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