scripts: check_init_priorities: fix device function name display

Device init pointers have been moved off struct init_entry and into
struct device in 766bfe7b, but check_init_priorities.py have not been
modified to check the new pointer so it's been showing NULL for all
devices since.

Fix it by searching down the right pointer for device init entries.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>