mlxbf-tmfifo: sparse tags for config access

mlxbf-tmfifo accesses config space using native types -
which works for it since the legacy virtio native types.

This will break if it ever needs to support modern virtio,
so with new tags previously introduced for virtio net config,
sparse now warns for this in drivers.

Since this is a legacy only device, fix it up using
virtio_legacy_is_little_endian for now.

No functional changes.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>