cnic: Simplify cnic_release().

Since unregister_netdevice_notifier() will replay the NETDEV_DOWN and
NETDEV_UNREGISTER_EVENTS, the cnic_dev_list will be cleaned up automatically.
The loop to cleanup the cnic_dev_list can be removed in cnic_release().

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>