vlan: cleanup the usage of vlan_dev_priv(dev)

This patch cleanup 2 points for the usage of vlan_dev_priv(dev):
* In vlan_dev.c/vlan_dev_hard_header, we should use the var *vlan directly
  after grabing the pointer at the beginning with
        *vlan = vlan_dev_priv(dev);
  when we need to access the fields of *vlan.
* In vlan.c/register_vlan_device, add the var *vlan pointer
        struct vlan_dev_priv *vlan;
to cleanup the code to access the fields of vlan_dev_priv(new_dev).

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>