ISIS: Fix warning: '@return' command used in a comment that is attached to a function returning void [-Wdocumentation]

Change-Id: I7f20311125e369cc90df808772323e8e98f8c94e
Reviewed-on: https://code.wireshark.org/review/5850
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>