Use ntoh24 instead of less readable and less correct code.
Remove redundant / wrong comments.
Add two small (helpful) comments :-)

Change-Id: I9c5451587d744a8418e6a90139b771a055e93387
Reviewed-on: https://code.wireshark.org/review/5366
Reviewed-by: Jörg Mayer <jmayer@loplof.de>