vfio/ccw: Remove mdev from struct channel_program

The next patch wants the vfio_device instead. There is no reason to store
a pointer here since we can container_of back to the vfio_device.

Reviewed-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/2-v4-8045e76bf00b+13d-vfio_mdev_no_group_jgg@nvidia.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>