s390/pai: rework paixxxx_getctr interface

Simplify the interface for functions paicrypt_getctr() and
paiext_getctr(). Change the first parameter from a pointer to a
structure to a pointer to a structure member. The other members
of the structure are not needed.
No functional change.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Acked-by: Mete Durlu <meted@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>