s390/zcrypt: make CPRBX const

Make this const as it is only used in a copy operation.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Harald Freudenberger <freude@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>