crypto: arm64/aes-neonbs - add return value of skcipher_walk_done() in __xts_crypt()

A warning is found by the static code analysis tool:
  "Identical condition 'err', second condition is always false"

Fix this by adding return value of skcipher_walk_done().

Fixes: 67cfa5d3 ("crypto: arm64/aes-neonbs - implement ciphertext stealing for XTS")
Signed-off-by: Yunfeng Ye <yeyunfeng@huawei.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>