[rilmodem] Ensure that *netreg_data is always set in callback data

This is based on commit 3b18c7773460b6156b68411795a4ceb0dad4725e
from Tony Espy in Canonical. Without setting the netreg value both
in manual and automatic network selection request ofono crashes
when selecting manual and then automatic selection.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>