vfio/ccw: cleanup some of the mdev commentary

There is no longer an mdev struct accessible via a channel
program struct, but there are some artifacts remaining that
mention it. Clean them up.

Signed-off-by: Eric Farman <farman@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>