s390/zcrypt: cleanup CPRB struct definitions

This patch does a little cleanup on the CPRBX struct
in zcrypt.h and the redundant CPRB struct definition in
zcrypt_msgtype6.c. Especially some of the misleading
fields from the CPRBX struct have been removed.

There is no semantic change coming with this patch.
The field names changed in the XCRB struct are only related
to reserved fields which should never been used.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Jürgen Christ <jchrist@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>