Bluetooth: Fix OBEX over L2CAP detection

There are two cases:
1. btl2cap -> btrfcomm -> btobex
2. btl2cap -> btobex

Case 2 is rare, so according to its name and to avoid confusion
I based on it.

Bug:10316
Change-Id: Ibeabeaf2f8376425460c56bad8fb980b460dd940
Reviewed-on: https://code.wireshark.org/review/3225
Reviewed-by: Evan Huus <eapache@gmail.com>