the code for calling register_callback is more readable and more
explicit now
Change-Id: I7a6a236d7f50b1ad391c1d49e3fb48f2580aa875
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/22564
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>