vdpa/mlx5: fix up endian-ness for mtu

VDPA mlx5 accesses config space as native endian - this is
wrong since it's a modern device and actually uses LE.

It only supports modern guests so we could punt and
just force LE, but let's use the full virtio APIs since people
tend to copy/paste code, and this is not data path anyway.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>