ASoC: fsl_spdif: Fix clock source for rxclk rate measurement

The rxclk rate actually uses sysclk, ipg clock for example, as its
reference clock to calculate it. But the driver currently doesn't
pass a correct clock source. So fix it.

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