i40e: Fix devlink port unregistering

Ensure that devlink port is unregistered after unregistering
of net device.

Reproducer:
[root@host ~]# rmmod i40e
[ 4742.939386] i40e 0000:02:00.1: i40e_ptp_stop: removed PHC on enp2s0f1np1
[ 4743.059269] ------------[ cut here ]------------
[ 4743.063900] WARNING: CPU: 21 PID: 10766 at net/devlink/port.c:1078 devl_port_unregister+0x69/0x80
...

Fixes: 9e479d64 ("i40e: Add initial devlink support")
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>