ASoC: WM8994: fix wrong value in tristate function

fix wrong value in wm8994_set_tristate func. when updating reg bits,
it should use "value", not "reg".

Signed-off-by: Qiao Zhou <zhouqiao@marvell.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@kernel.org