FC: fix dissection when packets are captured with Fibre Channel FC-2 encapsulation type

This is broken since ga9dd765

Bug: 10751
Change-Id: Id4f282d9e0104bfa2314e2348aedf31233a671bd
Reviewed-on: https://code.wireshark.org/review/5594
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>