This patch enables configuring the outer TPID for double VLAN.
Note that all other TPID values, for single VLANs or inner VLAN in the
QinQ case, are read only.
Signed-off-by: Beilei Xing <beilei.xing@intel.com>
Acked-by: Helin Zhang <helin.zhang@intel.com>