Always create a subtree for Mobility options, even if it doesn't have a specific dissection function.

Bug: 10618
Change-Id: Ic4d52485a6bd96f906b4a0d61b75a31bfad4e8f8
Reviewed-on: https://code.wireshark.org/review/5276
Reviewed-by: Michael Mann <mmann78@netscape.net>