[ethernet] Avoid false positive Coverity warning

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/ethernet.c

 	 * frames, without requiring a full LLC protocol layer.
 	 */
 	if ( eth_is_llc_packet ( ethhdr ) ) {
-		llc_proto = ( &ethhdr->h_protocol + 1 );
+		llc_proto = iobuf->data;
 		*net_proto = *llc_proto;
 	}