Disable the MDS Header ethertype preference.

If you encounter a frame with an all-zeroes ethertype it's very likely
due to buggy hardware or software. Don't try to dissect it as MDS
Header traffic.

Change-Id: Ie08d6aba2dcb82e4e49d6dbe96a2a3639e3cafc4
Bug: 8256
Reviewed-on: https://code.wireshark.org/review/5788
Reviewed-by: Gerald Combs <gerald@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>