ROS: Return dissected length from call_ros_oid_callback

Change-Id: I8d119cd3dc9ac0a724497400680a70f39fc21ede
Reviewed-on: https://code.wireshark.org/review/4641
Reviewed-by: Anders Broman <a.broman58@gmail.com>
Reviewed-by: Evan Huus <eapache@gmail.com>