ASoC: wm8731: harmless underflow in wm8731_put_deemph()

The code tries to verify that "deemph" is zero or one, but it fails to
account for that it can be negative.  It's harmless because negatives
are treated the same as one, but we should fix it just to silence the
static checker warning.

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>