crypto: aes_s390 - access .cip instead of .blk in cipher mode

The fallback code in cipher mode touch the union fallback.blk instead
of fallback.cip. This is wrong because we use the cipher and not the
blockcipher. This did not show any side effects yet because both types /
structs contain the same element right now.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>