qmi: Network registration via SSSP if possible

Doing the network registration via Set System Selection Preference is preferable because it doesn't override the currently set allowed mode, i.e. access technology.

https://lists.freedesktop.org/archives/modemmanager-devel/2020-July/007972.html

Signed-off-by: Sven Schwermer sven.schwermer@disruptive-technologies.com