virtio_ring: disable packed ring on unsupported transports

Currently, ccw, vop and remoteproc need some legacy virtio
APIs to create or access virtio rings, which are not supported
by packed ring. So disable packed ring on these transports
for now.

Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>