vlan: make __vlan_hwaccel_put_tag return void

Always returns the same skb it gets, so change to void.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>