HPSW: fix neighbor MAC Address

Use the wrong length for display mac address (6 and no the length of option)
Also fix update of item (don't update the root item)

Change-Id: I38cd1a6a045e797b01cd1a8de5add0949eaa0f02
Reviewed-on: https://code.wireshark.org/review/4950
Reviewed-by: Anders Broman <a.broman58@gmail.com>
(cherry picked from commit 9b37e1e207cb5dccc05d8dddae43a08335ac82ad)
Reviewed-on: https://code.wireshark.org/review/5160
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>