tg3: Refine tg3_vlan_rx_register()

tg3_vlan_rx_register() touches the hardware if netif_running() returns
false.  This patch fixes the problem.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>