net: vlan: fix comment for vlan_ethhdr->h_vlan_proto

After addition of 8021AD h_vlan_proto can be either ETH_P_8021Q or
ETH_P_8021AD.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: David S. Miller <davem@davemloft.net>