vlan: static functions

commit 6d4cdf47 (vlan: add 802.1q netpoll support) forgot to declare
as static some private functions.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Benjamin LaHaise <bcrl@kvack.org>
Signed-off-by: David S. Miller <davem@davemloft.net>