ASoC: wm2000: a couple harmless underflows

We want these to be zero or one, but by mistake we also accept negative
values.  It's harmless but we should still 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>