s390/atomic_ops: Let __atomic_add_const() variants always return void

Depending on MARCH_HAS_Z196_FEATURES __atomic_add_const() returns void or
the previous value before the atomic variant. Make sure that for both cases
void is returned so potential incorrect usage results in both cases in a
compile error.

Reviewed-by: Juergen Christ <jchrist@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>