crypto: s390/aes - fix typo in XTS_BLOCK_SIZE identifier

Fix a typo XTS_BLOCKSIZE -> XTS_BLOCK_SIZE, causing the build to
break.

Fixes: ce68acbc ("crypto: s390/xts-aes - invoke fallback for...")
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>