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: I38cd1a6a045e797b01cd1a8de5add0949eaa0f01
Reviewed-on: https://code.wireshark.org/review/4950
Reviewed-by: Anders Broman <a.broman58@gmail.com>