rijndael-s390x: fix checksum calculation in OCB decryption

* cipher/rijndael-s390x.c (aes_s390x_ocb_dec): Calculate checksum
after decryption instead of inlining.

OCB decryption was missing checksum inlining in 64 block loop.

• GnuPG-bug-id: T5356
• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>