ASoC: wm8955: harmless underflow in wm8955_put_deemph()

Valid values for "deemph" are zero and one, but we mistakenly allow
negative values as well.  It's harmless but we may as well clean it up.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>