ASoC: fsl-asoc-card: Get "extal" clock rate by clk_get_rate

ASoC: fsl-asoc-card: Get "extal" clock rate by clk_get_rate

On some platform(.e.g. i.MX8QM MEK), the "extal" clock is different
with the mclk of codec, then the clock rate is also different.
So it is better to get clock rate of "extal" rate by clk_get_rate,
don't reuse the clock rate of mclk.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Link: https://lore.kernel.org/r/1597047103-6863-1-git-send-email-shengjiu.wang@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>