The crypto_alloc_aead() function doesn't return NULL pointers, it returns
error pointers.  Fix the error checking.

Fixes: 06af76b4 ("crypto: spacc - Add SPAcc aead support")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>