ASoC: spear: spdif_out: Fix mute control

For controls registers with snd_soc_add_dai_controls snd_kcontrol_chip() returns
a pointer to the DAI, not to the CODEC.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>