ASoC: wm8904: harmless underflow in wm8904_put_deemph()

We try to check that "deemph" is zero or one, but because of an
underflow bug, it can also be negative.  It's fine, negative values are
handled as non-zero.  But it's messy and static checkers complain so
let's fix it.

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>