rtl8192du: Fix case where ethtype was never obtained and always be checked against 0

Zero-initializing ether_type masked that the ether type would never be
obtained for 8021x packets and the comparition against eapol_type
would always fail.

Reported-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>