ASoC: es8328: harmless underflow in es8328_put_deemph()

Valid values for "deemph" are zero and one but we accidentally allow
negative values as well.  It's harmless but it causes static checker
warnings and we may as well clean it up.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Brown <broonie@kernel.org>