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.de
Signed-off-by: Mark Brown <broonie@kernel.org>