vfio/ccw: Pass vfio_ccw_private not mdev_device to various functions

mdev_device should only be used in functions assigned to ops callbacks,
interior functions should use the struct vfio_ccw_private instead of
repeatedly trying to get it from the mdev.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/3-v4-cea4f5bd2c00+b52-ccw_mdev_jgg@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>