RTSE: Return dissected length from call_rtse_oid_callback

Change-Id: I6766135ddcdfe25d9bcc060bb2a47376abb36794
Reviewed-on: https://code.wireshark.org/review/4642
Reviewed-by: Anders Broman <a.broman58@gmail.com>
Reviewed-by: Evan Huus <eapache@gmail.com>