crypto: qce - fix wrong config symbol reference

The CONFIG_CRYPTO_DEV_QCE_SOFT_THRESHOLD symbol was renamed during
development, but the stringify reference in the parameter description
sneaked by unnoticed.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>