ASoC: wm8903: harmless underflow in wm8903_put_deemph()

Valid values for "deemph" are zero and one, but we accidentally allow
negative values as well.  It's harmless, but static checkers complain
and 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>