vfio/ccw: simplify the cp_get_orb interface

There's no need to send in both the address of the subchannel
struct, and an element within it, to populate the ORB.

Pass the whole pointer and let cp_get_orb() take the pieces
that are needed.

Suggested-by: Matthew Rosato <mjrosato@linux.ibm.com>
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>