of13: fix MTVlanVid serializer

It makes OFPVID_NONE (0x0000) impossible to be assigned to OXM_OF_VLAN_VID
as described in Table 13, openflow spec 1.3.4.But, Ryu always sets OXM_OF_VLAN_VID
with OFPVID_PRESENT bit during serialization in ofproto_v1_3_parser.MTVlanVid.
This patch fixes this problem.

Signed-off-by: Minoru TAKAHASHI <takahashi.minoru7@gmail.com>