ASoC: cros_ec_codec: allocate shash_desc dynamically

The wov_hotword_model_put() function has multiple large variables on
its stack, the largest of which is the result of SHASH_DESC_ON_STACK().
In total, this exceeds the warning limit for 32-bit architectures:

sound/soc/codecs/cros_ec_codec.c:776:12: error: stack frame size of 1152 bytes in function 'wov_hotword_model_put' [-Werror,-Wframe-larger-than=]

The function already has a dynamic crypto_alloc_shash() allocation, so
using kmalloc() for the descriptor is correct as well and does not
introduce any additional failure scenarios. With this, the stack usage
of wov_hotword_model_put() gets reduced to 480 bytes in my test
configuration.

Fixes: b6bc07d4 ("ASoC: cros_ec_codec: support WoV")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Tzung-Bi Shih <tzungbi@google.com>
Link: https://lore.kernel.org/r/20200507213405.1869430-1-arnd@arndb.deSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/cros_ec_codec.c

@@ -108,22 +108,23 @@ static int calculate_sha256(struct cros_ec_codec_priv *priv,
 			    uint8_t *buf, uint32_t size, uint8_t *digest)
 {
 	struct crypto_shash *tfm;
+	struct shash_desc *desc;
 
 	tfm = crypto_alloc_shash("sha256", CRYPTO_ALG_TYPE_SHASH, 0);
 	if (IS_ERR(tfm)) {
 		dev_err(priv->dev, "can't alloc shash\n");
 		return PTR_ERR(tfm);
 	}
-
-	{
-		SHASH_DESC_ON_STACK(desc, tfm);
-
-		desc->tfm = tfm;
-
-		crypto_shash_digest(desc, buf, size, digest);
-		shash_desc_zero(desc);
+	desc = kmalloc(sizeof(*desc) + crypto_shash_descsize(tfm), GFP_KERNEL);
+	if (!desc) {
+		crypto_free_shash(tfm);
+		return -ENOMEM;
 	}
+	desc->tfm = tfm;
+	crypto_shash_digest(desc, buf, size, digest);
+	shash_desc_zero(desc);
 
+	kfree(desc);
 	crypto_free_shash(tfm);
 
 #ifdef DEBUG