net: vlan: prepare for 802.1ad VLAN filtering offload

Change the rx_{add,kill}_vid callbacks to take a protocol argument in
preparation of 802.1ad support. The protocol argument used so far is
always htons(ETH_P_8021Q).

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>