ASoC: fsl_spdif: Fix incorrect usage of regmap_read()

We should not copy the return value into this val since it's supposed to
get the value of the register not the success result of regmap_read().
Thus fix it.

Signed-off-by: Nicolin Chen <Guangyu.Chen@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>