CVE-2024-42116 igc: fix a log entry using uninitialized netdev

In the Linux kernel, the following vulnerability has been resolved:

igc: fix a log entry using uninitialized netdev

During successful probe, igc logs this:

[ 5.133667] igc 0000:01:00.0 (unnamed net_device) (uninitialized): PHC added
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The reason is that igc_ptp_init() is called very early, even before
register_netdev() has been called. So the netdev_info() call works
on a partially uninitialized netdev.

Fix this by calling igc_ptp_init() after register_netdev(), right
after the media autosense check, just as in igb. Add a comment,
just as in igb.

Now the log message is fine:

[ 5.200987] igc 0000:01:00.0 eth0: PHC added