rijndael-s390x: fix AES256-XTS feature mask

* cipher/rijndael-s390x.c (_gcry_aes_s390x_setup_acceleration): Fix
AES256-XTS feature mask.

Implementation was checking for plain AES256 HW-acceleration instead of
AES256-XTS mode acceleration.

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