ASoC: wm8960: harmless underflow in wm8960_put_deemph()

We should only accept "deemph" values of zero and one, but by mistake we
accept negatives as well.  It's harmless but let's clean it up anyway.

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>