s390/virtio: add BSD license to virtio-ccw

The original intent of the virtio header relicensing
from 2008 was to make sure anyone can implement compatible
devices/drivers. The virtio-ccw was omitted by mistake.

We have an ack from the only contributor as well as the
maintainer from IBM, so it's not too late to fix that.

Make it dual-licensed with GPLv2, as the whole kernel is GPL2.

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>