crypto: sun8i-ss - use crypto_shash_tfm_digest() in sun8i_ss_hashkey()

Simplify sun8i_ss_hashkey() by using crypto_shash_tfm_digest() instead
of an alloc+init+update+final sequence.  This should also improve
performance.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>