elmi: Fix bug in message type coding

Bug: 10531
Ping-Bug: 10721
Change-Id: Ide6237afb71c79243347dedd1c744b3e97ff612e
Reviewed-on: https://code.wireshark.org/review/4542
Reviewed-by: Evan Huus <eapache@gmail.com>
(cherry picked from commit 57a7aca8c36254d29b71f819054b7a671b931ede)
Reviewed-on: https://code.wireshark.org/review/5432