crypto: sun8i-ss - fix a NULL vs IS_ERR() check in sun8i_ss_hashkey

The crypto_alloc_shash() function never returns NULL. It returns error
pointers.

Fixes: 801b7d57 ("crypto: sun8i-ss - add hmac(sha1)")
Signed-off-by: Peng Wu <wupeng58@huawei.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>