crypto: starfive - fix return value check in starfive_aes_prepare_req()

kzalloc() returns NULL pointer not PTR_ERR() when it fails,
so replace the IS_ERR() check with NULL pointer check.

Fixes: e22471c2 ("crypto: starfive - Add AES skcipher and aead support")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>