drm/virtio: fix error check

The >= compare op must happen in cpu byte order, doing it in
little endian fails on big endian machines like s390.

Reported-by: Sebastian Mitterle <smitterl@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200214080100.1273-1-kraxel@redhat.com