igc: fix a log entry using uninitialized netdev

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

Signed-off-by: Corinna Vinschen <vinschen@redhat.com>
Reviewed-by: Hariprasad Kelam <hkelam@marvell.com>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Naama Meir <naamax.meir@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>