staging: rtl8723au: OnAuth23aClient23a(): Clean up using struct ieee80211_mgmt

Use 80211_mgmt to determine offsets within the received frame. This
also removes a suspicious offset adjustment:

     offset = ieee80211_has_protected(hdr->frame_control) ? 4: 0;

which didn't make any sense, since it was only applied to determining
the auth, sequence number, and status, but wasn't applied to the
location of the IEs.

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