netlink: specs: ovs_vport: align with C codegen capabilities

netlink: specs: ovs_vport: align with C codegen capabilities

We started generating C code for OvS a while back, but actually
C codegen only supports fixed headers specified at the family
level right now (schema also allows specifying them per op).
ovs_flow and ovs_datapath already specify the fixed header
at the family level but ovs_vport does it per op.
Move the property, all ops use the same header.

This ensures YNL C sees the correct hdr_len:

   const struct ynl_family ynl_ovs_vport_family =  {
          .name           = "ovs_vport",
  -       .hdr_len        = sizeof(struct genlmsghdr),
  +       .hdr_len        = sizeof(struct genlmsghdr) + sizeof(struct ovs_header),
   };

Fixes: 7c59c9c8f202 ("tools: ynl: generate code for ovs families")
Link: https://patch.msgid.link/20250409145541.580674-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>