ASoC: cros_ec_codec: use sha256() instead of open coding

Now that there's a function that calculates the SHA-256 digest of a
buffer in one step, use it instead of sha256_init() + sha256_update() +
sha256_final().

Also simplify the code by inlining calculate_sha256() into its caller
and switching a debug log statement to use %*phN instead of bin2hex().

Acked-by: Tzung-Bi Shih <tzungbi@google.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Cc: alsa-devel@alsa-project.org
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Cheng-Yi Chiang <cychiang@chromium.org>
Cc: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Cc: Guenter Roeck <groeck@chromium.org>
Cc: Tzung-Bi Shih <tzungbi@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>